CHAPTER II. 4

FREE MESSMATES.

We meet with free messmates in various classes of the animal kingdom. They sometimes mount on the back of a neighbour, sometimes occupy the opening of the mouth, the digestive passages, or the exit for the excreta; at times they place themselves under the shelter of the cloak of their host, from whom they receive both aid and protection.

Among the vertebrates, there are few except fishes which merit a place here; it is only amongst these that we meet with species at the mercy of others, and dependent on acolytes, which are in every respect inferior to themselves.

An interesting messmate belonging to this first category is a fish of graceful form, named donzelina, which goes to seek its fortune in the body of a holothuria. Naturalists have long known it under the name of Fierasfer. It has a long body like that of an eel, entirely covered with small scales; and as it is quite compressed, it has been compared to the sword which conjurors thrust into their oesophagus. They are found in different seas, and all have similar habits. This fish is lodged in the digestive tube of his companion, and, without any 5 regard for the hospitality which he receives, he seizes on his portion of all that enters. The Fierasfer contrives to cause himself to be served by a neighbour better provided than himself with the means of fishing.

Dr. Greef, at present Professor at Marbourg, found at Madeira a holothuria of a foot in length, in which a vigorous Fierasfer lived in peace. Quoy and Gaimard, in the account of their voyage round the world, have remarked long since, that the _Fierasfer hornei_ is found in the _Stichopus tuberculosus_.

The holothuriæ seem to exist under very advantageous conditions in this respect, since we see Fierasfers, which are themselves tolerable gluttons, accompanied by Palæmons and Pinnotheres in the same animal. Professor C. Semper has seen holothuriæ in the Philippine Islands which bore a considerable resemblance, in this respect, to an hotel with its table d'hôte.

These singular fishes have been long noticed, but it was not till recently that their presence in a host so low in the scale as a holothurian could be explained.

But if naturalists are agreed as to the bond which unites these fishes to the holothuriæ, they do not agree as to the organs which they inhabit in their living hotel. Do they lodge in the digestive cavity of the holothuriæ, or do they inhabit the arborescent respiratory processes which open at the posterior extremity of the body? Until recently it was thought that it was in their stomach, but a doubt has arisen. Professor Semper, who has studied these animals with particular care at the Philippine Islands, had the curiosity to open the stomach of some of them, and found there, not the animals taken by 6 the holothuriæ, but the remains of its respiratory processess which they were in the act of digesting. Is it then merely a messmate? We must have more information on this point; and if it were not accidentally that the fierasfer swallowed the walls of the compartment in which he was lodged, he ought rather to take his place among parasites. Though it lodges in the respiratory processes, as the learned professor at Wurtzburg asserts, the fierasfer may also be a messmate after the fashion of so many others which inhabit the neighbourhood of the rectum, in order the more conveniently to snap up those animals which are attracted by the odour.

The fierasfers are not the only fishes which seek assistance from the holothuriæ; a species lives at Zamboanga, to which the specific name of Scabra has been given, and in the stomach of which, says Mons. Johannes Müller, usually lives a myxinoid fish, called _Enchelyophis vermicularis_. Unfortunately, we are not told in what part of the stomach it resides; for all is stomach in these animals.

It is less degrading for a fish to ask assistance from one in his own rank. The Mediterranean offers a curious instance of this. Risso saw at Nice, at the commencement of this century, the monstrous fish known under the name of _Beaudroie_ (the angler, or fishing-frog) lodging in its enormous branchial sac a fish of the family of the Murenidæ, the _Apterychtus ocellatus_. He is found there evidently under the condition of a messmate. Although the eels generally get their living easily, the Angler possesses fishing implements which are wanting in them, and when both of them are 7 immersed in the ooze, it carries on a fishery sufficiently abundant to enable it to share the spoil with others. This same angler lives in the northern seas, and there it harbours an amphipod crustacean, which until lately has escaped the vigilance of carcinologists. We shall speak of it further on.

Dr. Collingwood saw a sea anemone in the Chinese Sea, which was not less than two feet in diameter, and in the interior of which lodges a very frisky little fish, the name of which he could not tell.

Lieut. de Crispigny has observed a sea anemone (_Actinia crassicornis_) living on good terms with a malacopterygian fish, the _Premnas biaculeatus_. This fish penetrates into the interior of the anemone; the tentacles close round it, and it lives thus for a considerable time enclosed as in a living tomb. Mons. de Crispigny has kept these animals alive for more than a year, in order to make careful observations on them. A fish known by the name of _Oxybeles lumbricoides_ has been also found in the Indian Seas, which modestly takes up his quarters in a star-fish (_Asterias discoida_). Another case of _commensalism_ has been made known to us by Professor Reinhardt of Copenhagen. A siluroid of Brazil, of the genus _Platystoma_, a skilful fisherman, thanks to his numerous barbules, lodges in the cavity of his mouth some very small fishes, which were for a long time considered as young siluroids; it was supposed that the mother brought her progeny to maturity in the cavity of the mouth, as marsupials do in the abdominal pouch, or as some other fishes do. These messmates are perfectly developed and adult, but instead of living on the produce of their own labour, they prefer to instal themselves in the mouth of an obliging neighbour, and to take their 8 tithes of the succulent morsels which he swallows. This little fish has received the name of _Stegophilus insidiatus_. We see that in the animal world it is not always the great which take advantage of the little. Still, let us not be deceived; there are fishes in the latitude of the Island of Ceylon which really hatch their eggs in the cavity of the mouth, and we have seen some in the museum at Edinburgh, labelled with the name of _Arius bookei_. Louis Agassiz has made the same observation on a fish of the Amazon, which has also been recognised by Jeffreys Wyman. One fish wraps up its eggs in the fringes of its branchiæ, and protects them till they are hatched; another lays its eggs in holes hollowed out by itself in the steep banks of the river, and protects the young ones after they are hatched.

To hatch the eggs in the mouth is not more extraordinary than to hatch them in any other part of the body. The _Sygnathidæ_ hatch theirs in a pouch behind the anus; and it is a curious circumstance that the females do not undertake this duty. The males alone carry their progeny with them. This recalls to our recollection that curious example of the birds known under the name of _Phalaropes_, among which the males only hatch the eggs. The female of the cuckoo abandons her eggs, and entrusts them to the female of another bird.

The cuckoo suggests to us the mound-making Megapode and the Talegalla of Latham, both of which inhabit Australia; these birds deposit their eggs in an enormous mass of leaves or grass, which grows warm by decomposition, and the temperature of which is great enough to hatch them. The young ones when they come out of the egg are sufficiently 9 developed to be able to provide for their own wants, and to do without a mother's care.

To return to our animal messmates: let us notice the result of the observations of a learned and skilful naturalist who has rendered great services to ichthyology. Dr. Bleeker has described a still more remarkable association in the Indian seas; it is that of a crustacean, the _Cymothoa_, taking advantage of a fish known under the name of _Stromatea_; too imperfectly organized to fish for itself at large, but more skilful in snapping up all that comes within its reach, it makes its home in the buccal cavity of the Stromatea.

But of all crustaceans, the most cruel is the isopod named _Ichthyoxena_, which hollows out for itself and its female a large dwelling-place in the coats of the stomach of a cyprinoid fish. We will return again to these examples.

The _Physaliæ_, those charming living nosegays of the tropical regions, also give lodging in their cavities, and in the midst of their long cirrhi, to little adult and perfect fishes, belonging to the family of the _Scombridæ_, a family to which are attached the tunny and the mackerel. These sea-butterflies flutter away their indolent existence at the expense of their host. Voyagers tell us that they have seen them by dozens concealed in these animated festoons. Mons. Al. Agassiz has mentioned, in his illustrated catalogue, another fact, quite as extraordinary, observed in the Bay of Nantucket, in the United States; it relates to a nocturnal Pelagia (_Dactylometra quinquecirra_, Ag.) always accompanied, not to say escorted, by a species of herring. The two neighbours constitute together an 10 association which probably redounds to the advantage of both.

Without quitting our own sea-coast, we find an association of the same kind between young fishes (_Caranx trachurus_) and a beautiful medusa (_Chrysaora isocela_). This sea nettle often encloses several young specimens of Caranx, which we are surprised to see issuing full of life from the transparent bodies of these polyps. Indeed, it is not rare to find other fishes in the medusæ. Dr. Gunther, who has arranged with so much care the rich collection of fishes in the British Museum, has shown us some specimens of the _Labrax lupus_, and of the _Gasterosteus_, which had been obtained from the interior of different medusæ; and these associations have been also remarked by various distinguished observers, among whom we may mention Messrs. Sars, Rud. Leuckart, and Peach. The captain of the frigate _Jouan_, when in the Indian Sea, on October 26th, 1871, in 13° 20' N. lat., and 60° 30' E. long., that is to say, about 200 leagues to the west of the Laccadive Islands, saw, in very fine weather, the sea, which was at that time very calm, covered with medusæ, and the greater part of these were escorted by many little fishes of the genus _Ostracion_, the species of which he was unable to ascertain. It is probable that the school of medusæ set in motion certain animals which are eagerly sought after by the Ostracions.

The Pilot is a fish of which much has been recorded; fishing for it is one of the principal recreations of sailors during their long voyages. Some assure us that it snaps off the bait, without touching the murderous hook which threatens the shark; and as it never quits its companion, others have supposed that it lives on the morsels 11 abandoned by it. Neither of these suppositions is correct; and as the shark does not need its services to point out the danger, we must content ourselves with mentioning this curious association without endeavouring to explain it.

In fact, we have had the opportunity of examining many well-preserved specimens, the stomach of which contained potato parings, the carapaces of crustaceans, the _débris_ of fishes, marine plants (fuci), and a piece of _cut_ fish, which had evidently served as a bait. The pilot does not, therefore, live on the leavings of his companion, but on his own industry, and doubtless finds some advantage in piloting his neighbour. Through the great kindness of Dr. Gunther we have been able to make this interesting examination in the rich galleries of the British Museum. We desire to take this opportunity of expressing our gratitude to this learned man and to his illustrious colleagues, who have the direction of that vast establishment, which is ever open to those who labour for the advancement of science.

The pilot has sometimes been confounded with a very different fish, which does not merely remain in the neighbourhood of the shark, but establishes itself upon him, and moors himself to him by the aid of a particular apparatus, for a longer or shorter time; we may even say during the whole of the voyage. This is the Remora.

Is this fish the messmate of the shark to which he is attached? As in the case of the pilot, an examination alone could decide the question. We have opened at the British Museum the stomachs of several remoras of different sizes, and we have been able to ascertain that they also fish on their own account; their food was composed of morsels of 12 fish which had served as bait, of young fish swallowed whole, and of some remains of crustacea. The remora is simply anchored to his host, and asks from him nothing but his passage. He is contented, like the pilot, to fish in the same waters as the shark which transports him. Sailors, even now, are convinced that if any one of these remoras should attach itself to the ship, no human power could cause it to advance, and that it must of necessity stop. It is certain that the fishermen of the Mozambique Channel take advantage of this faculty, to fish for turtles and certain large fish. They pass through the tail of the remora a ring to which a cord is attached, and then send it in pursuit of the first passer-by which they consider worthy to be caught. This kind of fishing resembles in some degree the sport of hawking with falcons.

So extraordinary a being could not fail to attract the attention of those among the ancients who were students of nature. Pliny assures us that the remora was used in the preparation of a philtre capable of extinguishing the flames of love.

There must be many free animal messmates among insects, and entomologists should make them known; for example, many of them live with ants, as the _Pselaphidæ_ and _Staphylinidæ_. Certain hairs of these insects, it is said, secrete a sweet liquid of which ants partake greedily. If we may believe a skilful observer, Mons. Lespès, there are some among them, as the Clavigers, which in exchange for the services which they render are fed by the ants themselves. We may also mention the larvæ of the _Meloë_, which seem to live as parasites, and the true nature of which was so long unknown.

The females of the _Meloë_ lay their eggs near the ranunculus and 13 other plants whose flowers are regularly visited by bees. After these are hatched, the larvæ ascend into the flowers and wait patiently till a bee takes them on his back, and carries them into the interior of the hive. This insect was formerly known under the name of the bee-louse, but this appellation is improper, for the bee is not the host of the meloë, but simply its beast of burden. According to recent observations, flies perform the same office for _Chelifers_, and certain aquatic and land coleoptera for several kinds of acaridæ.

In the class of animal messmates we find also a coleopterous insect that lodges in a manner similar to the paguri, of which we shall presently speak. The female of the _Drilus_, a species allied to glowworms, attacks the snail, and when it has devoured it, instals itself in the shell, to pass through its metamorphoses; when necessary, it frequently changes its shell and chooses successively more spacious lodgings. Like a true Sybarite, the drilus weaves a curtain of tapestry before the entrance of its habitation, and remains there peaceably surrounded by the vestment of its youth.

Remarkable examples of free messmates are found more especially among crustaceans. It is well known that this class includes lobsters, crabs, prawns, and those legions of small animals which serve as the police of the sea-shore, purifying the waters of the ocean of all organic matters, which otherwise would corrupt them. They do not, like insects, shine with variegated colours; their forms are hardy and varied, and they are often pleasing on account of the singularity of their movements. Professor Verrill has recently studied some of 14 these creatures, and has clearly shown how interesting they are, not only to naturalists, but to people in general.

Crustaceans and worms furnish the greatest number of paupers and infirm individuals; and a great many of them need the continual assistance of their neighbours to enable them to get their living. While other animals advance towards perfection as they grow older, it is far different with many crustaceans, and we should be tempted to refer to the vegetable kingdom many of them at the very period when they are approaching the adult condition. Cuvier placed all the class of cirrhipedes among the mollusca, and the lernæans among the worms. Many of these animals which are but indifferently adapted to live without help from others, have recourse to benevolent neighbours; from one they seek only shelter, from another a part of his booty, from a third both an asylum and protection. They are often reduced to a mere skin; everything else has disappeared, and there remains no proper organ except that which is necessary for the reproduction of the species. Corpulent, blind, impotent, legless cripples, their existence is more precarious than that of those miserable mutilated beings found in our cities; they only live on the blood of the neighbour which gives them an asylum. Yet when they first quit the egg they are all free; they frisk, they swim with the rapidity of lightning, and at the close of life we find them deformed, and crouched in some living refuge, as if a foul leprosy had atrophied within them all the organs which served as a means of communication with the outer world. Parasites and messmates, furnished at first with the same kind of limbs and the same habits, can sometimes only be distinguished from 15 each other when we have made our observations on them in their first swaddling clothes. The child has given a clue to the history of the old man.

We will not examine these animals in all the details of their private life, and yet we are strongly tempted to confess to our readers some of the indiscreet acts of which we have been guilty, in watching them while changing their dress. Notwithstanding their shyness and their desire to escape observation during the moulting period, we have more than once made observations on them while quitting their garment which has become too small. The old tunic generally splits down the back, and falls off all in one piece as it gives the animal egress. The crustacean is extended quite soft and supple by the side of its rigid carapace.

Of all the free crustacean messmates, one of the most interesting, though among the smallest of them, is a tiny crab, about as large as a young spider, which lives in mussels, and which has been often accused, though evidently wrongfully, as the cause of the indisposition so well known by those who are fond of this mollusc. Very many of them have been seen within the last few years, and yet accidents have been very few. The mussels themselves are guilty; they produce on some persons an injurious effect, through _idiosyncracy_. We have at least a word to serve as an explanation, and at present we must content ourselves with it.

Under what conditions do those crabs, called by naturalists Pinnotheres, and which we do not find elsewhere, inhabit mussels? Are they parasites, pseudo-parasites, or messmates? It is not a taste for voyaging which tempts them, but the desire of having always a secure 16 retreat in every place. The pinnothere is a brigand who causes himself to be followed by the cavern which he inhabits, and which opens only at a well-known watchword. The association redounds to the advantage of both; the remains of food which the pinnothere abandons are seized upon by the mollusc. It is the rich man who instals himself in the dwelling of the poor, and causes him to participate in all the advantages of his position. The pinnotheres are, in our opinion, true messmates. They take their food in the same waters as their fellow-lodger, and the crumbs of the rapacious crabs are doubtless not lost in the mouth of the peaceful mussel. There is no doubt that these little plunderers are good lodgers, and if the mussels furnish them with an excellent hiding-place and a safe lodging, they themselves profit largely by the leavings of the feast which fall from their pincers. Little as they are, these crabs are well furnished with tackle, and advantageously placed to carry on their fishery in every season. Concealed in the bottom of their living dwelling-place (a den which the mussel transports at will) they choose admirably the moment and the place to rush out to the attack, and always fall on their enemy unawares. Some of these pinnotheres live in all seas, and inhabit a great number of bivalve molluscs. The northern seas contain a large species of Modiola (_Modiola Papuana_) which is especially found in deep and almost inaccessible parts, and which always encloses a couple of pinnotheres about the size of a hazel-nut. We have opened hundreds of these modiolæ, and we have never met with any without their crabs. We have long since deposited some specimens of these 17 pinnotheres in the galleries of the Natural History Museum at Paris.

The large mussel, which furnishes fine pearls (_Avicula margaritifera_), lodges also pinnotheres of a particular species by the side of another messmate more allied to a lobster than a crab. It is not even impossible that these crustaceans, with other messmates or parasites, contribute to the formation of pearls, since these gems, so highly prized in the fashionable world, are only the result of vitiated secretions, and are usually the result of wounds.

We also meet with a little crab (_Ostracotheres tridacnæ_, Ruppel) in the acephalous mollusc, whose immense shell sometimes serves as a vessel for holy water; and it lives doubtless in many other bivalves which have not yet been examined.

Dr. Léon Vaillant has written a very interesting memoir on the Tridacnæ, and informs us that the crab takes shelter in their branchial chamber. Therefore, since the molluscs live only on vegetable substances, while the Ostracotheres feed entirely on animal matter, Mons. Vaillant supposes that the latter take their choice of the food as it enters, and seize on its passage that which suits them best. Mr. Peters, during his abode on the coast of Mozambique, studied a great many of these acephala and pearl-mussels, and found their interior inhabited by three crustacean decapods, a pinnothere, and two macrouræ allied to the _Pontonia_, to which he has given the name of _Conchodytes_; the _Conchodytes tridacnæ_ inhabits the _Tridacna squamosa_; the _Conchodytes meleagrinæ_, as its specific name indicates, lives in the shell of the pearl-mussel.

Professor Semper has recently observed pinnotheres in holothurians at 18 the Philippine Isles, and Mons. Alphonse M. Edwards has described some from New Caledonia (_P. Fischerii_); so that these little crabs, the friends of the molluscs, are known in both hemispheres.

Do not these conditions seem to authorize the conclusion that the same thought has presided over the appearance of all living creatures; that they have all come into existence, not according to the chance arrangement of surrounding media, but according to the laws established from the very origin of all things?

The shell which lodges both these pinnotheres, in the Mediterranean as well as the Atlantic, is a large acephalous mollusc, known under the name of _Jambonneau_ (a small ham or gammon), and which, according to Aristotle, harbours two different kinds of messmates. This illustrious natural philosopher also described a Pontonia (_Pontonia custos_, Guérin--_P. Pyrrhena_, M. Edw.) about an inch and a half long, of a pale rose colour, more or less transparent, and which lives with its companion, the pinnothere, in the cavity of the _Pinna marina_. This is the same animal which a naturalist of the last century named the _Cancer custos_.

We have wished to ascertain whether Pliny knew these crustaceans. He has spoken of them in the following terms:--"The Chama is a clumsy animal without eyes, which opens its valves and attracts other fishes, which enter without mistrust, and begin to take their pastime in their new abode. The pinnothere seeing his dwelling invaded by strangers, pinches his host, who immediately closes his valves, and kills one after another these presumptuous visitors, that he may eat them at his leisure."

Cuvier did not believe that the pinnothere brought any food to the 19 mollusc, since the latter, in his opinion, lives entirely on sea-water.

Other zoologists regard the pinnothere as an intruder whom chance has brought into this mysterious position. Others again consider mussels as acquaintances possessed of a very curious disposition, and that having no eyes, they have interested in their fate this little crab, which is perfectly provided with eyesight. In fact, in common with other crustaceans of his species, he carries on each side of his carapace, at the end of a movable stalk, a charming little globe, provided with some hundreds of eyes, which he can direct upon his prey, as the astronomer turns his telescope on any point of the firmament. These later naturalists consider, in fact, their crab as a living journal which supplies his host with the news of the day. Rumphius, a Dutchman, the first who described the animal of the nautilus, also understood the habits of pinnotheres. In his "Amboinche Rariteit Kamer," published in 1741, he says that these crustaceans inhabit always two kinds of shellfish, the _Pinna_ and the _Chama squamata_. According to him, when these molluscs have attained their growth, one pinnothere (one only at least in the Chama) lives in their interior and does not abandon its lodging till the death of its host. Rumphius regards this crustacean as a faithful guardian, fulfilling the duties of a door-keeper. In 1638 he found actually two sorts of keepers: by the side of a Brachyuron, carrying an embossed buckler, slender in front, he discovered a Macrouron of the length of his finger-nail, of a yellowish orange colour, semi-transparent, with white and very slender claws. It is without doubt the same animal 20 that Mons. Peters, of Berlin, found on the coast of Mozambique, and of which we have spoken before.

A little crab is known to live near the coast of Peru (_Fabia Chilensis_, Dana), which exists under somewhat different conditions. He chooses, not a bivalve mollusc, but a sea-urchin (_Euriechinus imbecillus_, Verrill), and lodges in the intestine, near its termination, so as to seize as they pass by all those living creatures which are attracted by the odour. Doubtless, the delicacy of our sense of smell is disgusted by such a mode of seeking food; but this predilection may have a reason with which we are not acquainted. There are a considerable number of other species which live under similar conditions.

On the coast of Brazil, my son found two couples of crabs in the tube of a very long annelid, narrow at the ends, and wide in the middle. The tube was too small at the end to allow them to escape. These crustaceans had, no doubt, penetrated thither before they had attained their full size.

A crab of the family of the Maidæ conceals itself in the substance of a polypidom very common in the Viti Islands, in company with a gasteropod mollusc, and both of them assume the exact colour of the polypidom. This is a new kind of _mimicry_. This crab is known by the name of _Pisa Styx_, the gasteropod is a _Cypræa_, the polyp is the _Melithea ochracea_. A decapod crustacean, the _Galathea spinirostris_, seeks for a _Comatula_, the colour of which it exactly imitates, and with which it lives on the most friendly terms.

The holothuriæ, of which we have already spoken, appear to afford an abode to many animals: independently of the _Fierasfer_, the 21 _Holothuria scabra_ of the Philippine Islands regularly lodges in its interior a couple, and sometimes, though rarely, a greater number of pinnotheres belonging to two distinct species. They choose this domicile at an early period, and must be highly delighted with this obscure abode, since they are seen no more, and when they have once entered never quit this living cavern. This observation is due to Professor Semper, who has made us acquainted with so many curious facts of the China Sea and the Pacific Ocean. In the midst of the slender branches of a coral of the Sandwich Islands, the _Pæcilopora cæspitosa_ of Dana, there lives a little crab (_Hopalocarcinus marsupialis_, Stimpson), which is at last completely enclosed by the vegetation of the coral. It only keeps up sufficient communication with the exterior to enable it to procure food. The coral, however, furnishes it nothing but a resting-place in the midst of its tissues.

Among the Philippine Islands, also, a brachyurous crustacean lives in the branchial cavity of one of the _Haliotidæ_, and another on the body of a holothuria. On the coasts of Brazil, F. Müller, during his abode at Desterro, saw some _Porcellanæ_ inhabiting star-fish, not as parasites, as had been supposed, but as true messmates. A crustacean possessed of but little generosity is the _Lithoscaptus_ of Mons. Milne-Edwards. Provided with beak and claws for the purpose of attack, it instals itself, sad to say, in the pantry of a medusa, and instead of making use of its own weapons, takes advantage of the perfidious nematocysts of its acolyte, in order to live quietly at his expense.

Under the name of _Asellus medusæ_, Sir J. G. Dalyell has made 22 us acquainted with another messmate of the medusæ which greatly resembles an _Idothea_.

Another kind of commensalism is that of the Dromiæ. These crabs are of the ordinary size, and lodge, from their earliest youth, under a growing family of polyps, which increases with them. This colony has for its principal foundation a living Alcyonium, which covers the carapace, and as it develops, adapts itself perfectly to all the inequalities of the cephalothorax; one might consider it an integral part of the crab. Sertulariæ, Corynes, Algæ, develop themselves on this Alcyonium, and the Dromia, masked by this living rock which it carries on its shoulders like the fabled Atlas, marches gravely in pursuit of her prey. She has no fear of arousing the attention of her enemies. The greatest vigilance cannot prevent the sudden attack of these dangerous neighbours. There is in the Mediterranean a species which sometimes comes to our coast. They are also known in the Indian Seas and in the Northern Pacific. Rumphius named the dromia _Cancer lanosus_; it is, said he, a crab which carries grass or moss on its back. It is also mentioned by Renard. Dana has observed a sea-anemone covering a crab in the same manner as the Alcyonium does the dromia, and which is not less dangerous. The mode of life of this anemone has procured for it the name of _Cancrisocia expansa_. In the north of California, a crab (_Cryptolithoides typicus_) covers itself in the same manner with a living cloak which hides it from view, and under cover of which it surprises those whom it attacks. It has already cleared the ground of its prey before any alarm has been given to the neighbourhood.

We should perhaps speak here of an association of another kind, the 23 nature of which it is difficult to ascertain; I refer to the little crab, the Turtle Crab of Brown, which is met with in the open sea on the carapace of turtles, and sometimes on sea-weeds. It may be supposed that it takes advantage of the carapace of its neighbour, in order to transport itself at little expense into different latitudes, and it is asserted that the sight of this crustacean gave confidence to Christopher Columbus, eighteen days before the discovery of the New World. Besides this animal, a whole society chooses this movable habitation: in addition to the cirrhipedes we also find the _Tanaïs_, which is not, however, condemned to live there always.

The macrourous decapods are more rarely found as messmates, but still a Palæmon is sometimes seen on the body of an Actinia, according to Semper, and another in the branchial cavity of a Pagurus. But that which is more generally known, is the presence in the _Euplectella aspergillum_ of the palæmon which lodges in this fairy palace. It is probable that the Euplectella of the Atlantic, recently observed near the Cape Verd Islands by the naturalists on board the _Challenger_, also conceals this crustacean in its interior. We may also allude here to the _Hypoconcha tabulosa_, a crab whose carapace is too soft to allow it to venture out undefended, and which covers itself with the shell of a bivalve mollusc.

Among the various associations of this kind, none is more remarkable than that of the soldier-crabs, so abundant on our coasts, and called by the names of _Bernard the Hermit_ and _Kakerlot_ by the Ostend fishermen. It is well known that these crabs are decapod crustaceans, very like miniature lobsters, which lodge in deserted shells, and 24 change their dwelling-place as they grow larger. The young ones are content with very little habitations.

The shells which give them shelter are such as have been shed, which they find at the bottom of the sea, and in which they conceal their weakness and their misery. These animals have an abdomen too soft to bear the dangers which they meet with in their warfare, and that they may be less exposed to the claws of their numerous enemies, they take shelter in a shell which serves at the same time both as a dwelling and a buckler. Armed cap-à-pie, the soldier-crabs march boldly on the enemy, and know no danger, since they always have a secure retreat.

But this animal does not live alone in this asylum. He is not so much of an anchorite as he appears to be, for by his side an annelid usually instals himself as a messmate, which forms with the Pagurus one of the most terrible associations that are known. This annelid is a long worm, like all the nereids, whose supple and undulating body is armed along its sides with arrows, lances, pikes, and poniards, the wounds of which are always dangerous. It is a living panoply which glides furtively into the enemy's camp without giving the alarm.

When a pagurus is on the march it resembles a nest of pirates, who never cease their exploits till all has been ravaged around them. This shell is so innocent in its appearance, that it introduces itself everywhere without provoking the least suspicion. It is usually covered with a colony of Hydractiniæ, and in the interior, Peltogasters, Lyriopes, and other crustaceans often establish 25 themselves. The paguri are not messmates of an ordinary kind, for they inhabit only a deserted shell. They are spread over all seas. They are found in the Mediterranean, the Northern Sea, on the coasts of the Pacific, of New Zealand, and of the East Indian islands: thirty species and even more have been inserted in the catalogue of crustaceans.

Naturalists have given the name of _Cenobitæ_ to some pagurians inhabiting the seas of warmer latitudes; these have an abdomen like the pagurus, antennæ like the _Birgus_, and like it they inhabit shells. The _Cenobita Diogenes_ is a species found in the Antilles.

Other pagurians, the _Birgi_, grow very large, and conceal their abdomen no longer in a shell, but in the crevices of the rocks, as lobsters do at the moulting time, to protect their body while deprived of their defensive armour. In the East Indies they remain on land, and even climb into trees. They have so much strength in their pincers, that Rumphius relates of one of these crustaceans, that, while stretched on a branch of a tree, it raised a goat by the ears.

Side by side with the pagurians which instal themselves in a shell with thick and completely opaque walls, we recognize crustaceans of the order of amphipods, the _Phronimæ_, which choose for themselves not an abandoned hovel, but a veritable crystal palace, and take possession of it without inquiring whether or no it is inhabited. The daylight penetrates through the walls of their dwellings, and it can scarcely be discerned in the water whether or no their body is protected by a covering. They usually take the dwelling of a Salpa, a Beroë, or a Pyrosoma, and from within this lodging they give 26 themselves up to the pleasures of fishing.

The _Phronima sedentaria_ which lodges with the salpa seems to be scattered over the warm seas of both hemispheres. For the honour of the species, the females alone seek the assistance of their neighbours, without at the same time abandoning their characteristic robe. The sexes differ little from each other except in size, in the abdomen, and in the antennæ. Maury has described certain amphipod crustaceans which also inhabit the Salpæ.

Another phronima described by Professor Claus, the _Phronima elongata_, lives in the same manner; but instead of occupying a living house, it generally seeks an empty lodging, in which it establishes itself like a pagurus.

The "Bernard the Hermit" of the Marseillaise fishermen, the _Pyades_, becomes the messmate of an anemone which Dugès has called _Actinia parasitica_. According to the observations of the learned professor at Montpelier, the mouth of this anemone is always situated opposite to that of the crustacean, to take advantage of the morsels which escape from his pincers. Both of them profit by this association; and the opening of the shell is prolonged by a horny expansion furnished by the foot of the actinia.

On the coast of England lives another soldier-crab (_Pagurus Prideauxii_), which has as its principal messmate a sea anemone called _Adamsia_, which Mons. Greeff found at the island of Madeira. This pagurus is especially remarkable for the good understanding which exists between himself and his acolyte--he is a model Amphitryon. Lieut.-Col. Stuart Wortley has watched it in its private life, and thus relates the result of his observations: this 27 animal after he has fished, never fails to offer the best morsels to his neighbour, and often during the day, ascertains if it is not hungry. But more especially when he is about to change his dwelling, does he redouble his care and his attention. He manoeuvres with all the delicacy of which he is capable, to make the anemone change its shell; he assists it in detaching itself, and if by chance the new dwelling is not to its taste, it seeks another until the _Adamsia_ is perfectly satisfied. This association is not confined to the union of a decapod with a nereid and an actinia; a curious cirrhipede often establishes itself on the body of the pagurus, and on the outside of the shell we generally find a colony of polyps, of a rose or yellow colour, which extend like a living carpet round this habitation. Thirty-six years ago we have given the name of _Hydractinia_ to these polyps, which were till then entirely unknown to naturalists, and which form habitually a double overcoat for the paguri, if I may employ the expression of my learned colleague, Mons. Ch. Desmoulins.

In the Mediterranean lives the _Perella di mare_ of the Italian fishermen, the _Reclus marin_ of the Marseillaise; this Alcyonium ought, by its manner of life, to be placed near the Hydractiniæ, and has been carefully studied by Mons. Ch. Desmoulins. It is the _Alcyonium_ (_Suberites_) _domuncula_ of Lamarck and Lamouroux.

The abdomen of these paguri is not only sheltered in a shell, but habitually visited by isopod crustaceans, described under the names of _Athelea_, _Prosthetes_, and _Phryxus_, which have entirely lost the livery of their order.

In the same association we also find the _Liriope_, a little isopod 28 crustacean, of which much has been said, but which for a long time obstinately resisted all attempt at observation.

This latter personage is an isopod crustacean, of moderate size, which chooses the Peltogaster as a place of abode, after having undergone a very curious regressive metamorphosis. In fact, the young lyriope has at first its little feet like other isopods, but in the adult state, the female loses her antennæ, and changes her buccal as well as her branchial appendages, so as to assume a different appearance. Several naturalists have already endeavoured to give the life-history of this singular Bopyrian. The illustrious Rathke of Königsberg discovered it; Professor Lilljeborg, of the University of Upsal, gave the first account of it; and finally Professor Steenstrup of Copenhagen made known its true origin. In short, the Lyriopes are Bopyrian Isopods, living on cirrhipedes (Sacculinideæ) as real messmates, if not as parasites; the male preserves his dignity and his prestige, but the female strips herself of all the attributes of her sex, and descends to the lowest degree of servitude.

Faujas de Saint-Fond has mentioned a fossil hermit-crab as found in the mountain, St. Pierre de Maestricht; but he called by this name a crustacean of the genus _Callianassa_ and not a pagurus. These _Callianassæ_ are always completely isolated in the chalk, and it is probable that they have no other domicile than the sand or ooze at the bottom of the sea, in which they hollow out galleries for themselves. Lobsters act in the same manner after moulting. The _Gebiæ_ live like the Callianassæ, hidden in the mud. The _Limnaria lignorum_ and the 29 _Chelura terebrans_ dig out a retreat for themselves in wood, like the Teredines.

We have just seen that the higher crustaceans, with their well-mounted eyes, their enormous antennæ, and their formidable pincers, are not all of them the great lords they pretend to be; more than one of them has to hold out its hand and to accept humbly the assistance of its neighbours.

In the group of isopod crustaceans we find many necessitous beings, which, too proud to ask for food, are contented to take their place on some fish which is a good swimmer, which they abandon as soon as their interest demands it; if their host conducts them to regions that do not suit them, or if they have otherwise to complain of him, they give him up, and begin their maritime peregrinations with a fresh colleague. They always preserve all their fishing tackle and their sailing gear, and the female does not change her dress any more than the male. We have to notice that these crustaceans often identify themselves so entirely with their host that they seem to be a portion of him, and even to assume his peculiar colour. This is not a sign of servility, but a means of passing unobserved, and of escaping from the sight of the enemy that is watching them. Naturalists have given the name of _Anilocræ_ to some of these free messmates.

Any one who has remained for some time on the coast of Brittany, especially at Concarneau, and who does not look with indifference on the many superb fishes which are taken every day, cannot fail to have been struck with the presence of a rather large crustacean, which clings to the sides of several kinds of _Labra_, especially 30 the smaller species. This crustacean is an Anilocrian so common that we can scarcely imagine it to have escaped the attention of any naturalist. Nevertheless, no work makes mention of the regular attendance on the Labra by the Anilocra, which bears, we know not why, the specific name of _Mediterranean_. Rondelet was probably acquainted with it, when he spoke of the fish-lice, which do not derive their birth from these fishes, but from the sea mud. We often see males by the side of females on the same individual.

Some years ago a school of large cetaceans, known under the name of Grindewhalls or Globicephalæ were pursued in the Mediterranean, and those which were captured contained in the cavity of their nostrils, isopods closely allied to the _Cirolana spinipes_, if not identical with it. Till then the isopods had only been found on sea fishes; fresh-water fish are not, however, entirely exempt; in fact, a species of OEga (_OEga interrupta_ of Martens) has just been found on the skin of a fresh-water fish of Borneo, the _Notopterus hypselonotus_. This same genus includes a species (_OEga spongiophila_) which lives in the magnificent sponge, the _Euplectella_. We know also a certain number of isopods which prefer the interior of their neighbour's body, and instal themselves in the cavity of the mouth, either to fish at the same time as their host, or to seize the food on its passage; others are of such a cruel nature, that they make no scruple to establish themselves in the stomach of a peaceable white fish. Without injuring any important organ, they penetrate in couples between the intestines, and, concealed in this retreat, they seize by the narrow entrance door, 31 which they keep half open, all the little animals which are sufficiently bold to pass by. The cruelty of these beings knows no bounds. To instal themselves conveniently, they pierce the body of their host, skilfully open his stomach, and live there as Sybarites; their lodging is in future assured to them, and their fate is bound up with that of their host. Dr. Herklots, who has unfortunately been recently lost to science, communicated in 1869, to the Academy of the Netherlands, a very interesting memoir on two crustaceans of a new species, the _Epichtys giganteus_, which lives on a fish of the Indian Archipelago, and the _Ichthyoxenus Jellinghausii_, which lodges in a fresh-water fish of the Island of Java. It is to the latter that we refer here, and it seems that in this species we are approaching the limits at which commensalism commences.

The _Cymothoes_ constitute another category of very interesting Isopods; they lodge with their female in the cavity of a fish's mouth. Dr. Bleeker, who has so successfully explored the Indian seas, obtained more than twenty species of these; but unfortunately he has not made a note of the fishes which harbour them. He has, however, made one exception with regard to a fish from the roadstead of Pondicherry, which is two feet long, and is called a Bat. It is known to naturalists under the name of _Stromatea Nigra_; its flesh is much esteemed, and it carried in its mouth a Cymothoe called by Dr. Bleeker _Cymothoe Stromatei_. A cymothoe has also been observed in the mouth of an Indian Chetodon. De Kay found one in a Rhombus in the United States, and De Saussure saw another at Cuba; and lately, Mons. Lafont discovered one in the Bay of Arcachon, on the _Boops_, and on the 32 _Trachina vipera_. These cymothoes are about fifteen millimetres in length, and often fill all the cavity of the mouth. The most curious of all is that which is found in the mouth of the flying-fish, a kind of herring with elongated fins, which it uses as wings to rise into the air, when too closely pursued in the water. My son, when examining these fishes, in his passage from Cape Verd to Rio de Janeiro, found in the cavity of their mouth an enormous female, firmly wedged in the branchial arches, with its head inclined outwards, and the male, which was rather smaller, installed at her side. Their dwelling thus by pairs, as well as the entire conformation of the animal, plainly shows that these crustaceans make themselves at home, and live as true messmates. Cunningham has given them the name of _Ceratothoa exoceti_. A short time since, these Cymothoes were only known on marine fishes, but it appears from recent observations, that fresh-water fish are far from being exempt from them. Mons. Gertsfeld has recently noticed some on the _Cyprinus lacustris_ of the river Amour, and another in the Rio Cadea in Brazil, on a _Chromida_. Other isopods also resort to fishes, and to animals of their own class, but they live as true parasites, and change their form as soon as they have chosen a resting-place. We shall return to this subject again. Some which are very common on prawns, are known under the name of Bopyrus.

An interesting division of amphipods have received the name of _Hyperinæ_. These crustaceans generally swim with facility, but walk with difficulty. They therefore usually have recourse to fishes, or even to medusæ, in order to gain support. We find on our own coasts the _Hyperina Latreillii_, lodged in the superb _Rhizostoma_, which 33 regularly appears in the later season of the year on the coast of Ostend; and a long time since, in 1776, O. F. Müller gave to a species of this genus the name of _Hyperina medusarum_. Mr. Alexander Agassiz once found a _Hyperina_ on the disc of an _Aurelia_. The medusa, when extended, forms for them a balloon with its parachute, which supports and conveys them with greater or less rapidity. Professor Möbius has but lately remarked the presence of _Hyperina galba_, Mont., in the _Stomobrachium octocostatum_, Sars, a small species of medusa which appears in the Bay of Kiel in October and November. This naturalist supposes that these messmates at first inhabited the _Medusa aurita_, and then migrated into this species.

Besides these, there are _Gammari_, which, according to Semper, live in the _Avicula meleagrina_ (pearl mussel), and are perhaps the principal manufacturers of fine pearls. The immense buccal cavity of the fishing-frog (_Lophius piscatorius_) is the abode in the Mediterranean of an _Apterychta_, and in the Northern Ocean of a curious amphipod of the ordinary size of the _Gammarus_, which takes a voyage without expense, and with no fear of wanting provisions. My son discovered it at Ostend, and proposes the name of _Lophiocola_ to distinguish it. The Gammari give lodging themselves to a great quantity of parasites, which they must introduce into the bodies of those to whom they serve as food. It has been long known that whales have lice, to which naturalists have given the name of Cyami. They are found on the whales of both hemispheres, and on some other cetaceans. It is very remarkable that they are seen on the true whales of 34 the north and of the temperate regions, on the _Megaptera_, and on several _Catodonta_, and that none are found in the _Balenoptera_. Mr. Dall has just noticed some on the singular _Grey Whale_ of California. In general, we may say that each cetacean which harbours them, has its own species. Are they parasites or messmates? If we are to believe Roussel de Vauzème, they feed on the skin itself of the whale, the remains of which, it is said, are found in their stomach. According to this naturalist, the parts of the mouth are not adapted for suction, and the stomach contains ruminating apparatus. We think that a fresh examination is necessary before this question can be determined. The _Cyami_ seem to us to live on the whale, as the _Arguli_ and the _Caligi_ do on fish; and if these living creatures derive their nourishment only from the mucous products secreted by the skin, we may ask whether they ought not to be classed in a separate category, for they ought not to figure on the list of paupers. We have found the orifice of the _Tubicinella_ covered with cyami of every age, and their abundance in this place seems to indicate that their food was not supplied to them by the skin of their host. Mons. Ch. Lutken has recently published a very interesting monograph on these curious animals; according to him the _Cyamus rhytinæ_, which was thought to proceed from a piece of the skin of a _Stellerus_, appears to have been found on the skin of a whale.

The Picnogonons, the nature as well as the kind of life of which has been so long time problematical, deserve to be ranked among messmates, at least during their youth; in fact, after being hatched, they live on the _Corynes_, the _Hydractiniæ_, and other polyps, while at a later 35 period they frequent molluscs or higher classes; Allman mentions the case of a _Phoxichilidium coccineum_ lodged in a _Syncoryne_.

There are, perhaps, many other crustaceans which, placed among messmates, like the _Pandarus_ and others, would have a right to claim a further inquiry. It is a fact that they are never seen except on the skin of their host, where they are always visible, preserve their colours entire, and never change their costume for the undress of a parasite. The _Pandari_ live especially on the _Squalidæ_. Some which are found in our seas are of rare elegance of form. We must, perhaps, place among messmates the crustacean which Siebold found in the Adriatic, at Pola, on the belly of the worm _Sabella ventilabrum_, and it is not impossible that the _Staurosoma_ observed by Will on an actinia, should have its place here rather than among the parasites.

A Rotifer without vibratory ciliæ, the _Balatro calvus_ of Claparède, lives as an epizoon on the same annelids which lodge the Albertia in their interior. The Darwinists, observes Claparède, will not fail to remark the presence of these Rotifers of the genus Albertia in the interior of the animal, and of the genus Balatro on the exterior. The parasite Balatro, like a shadow, never quits his Mecænas, says the learned naturalist of Geneva; who has observed it on the _limicolous Oligochæts_ of the Seime, in the Canton of Geneva.

The _Nebalia_ of Geoffroy is an interesting crustacean, abundant on the coast of Brittany. This charming animal gives lodging habitually to a messmate which Mons. Hesse considered as an animal allied to the 36 _Histriobdellæ_, but which is only an imperfectly described Rotator. We believe that it is the same animal to which Professor Grube has given the name of _Seison nebalia_. It appears to assume the aspect of the Histriobdellæ, and may perhaps be adduced as an example of mimicry.

The molluscs, whatever their name may imply, are those which show the most independence among all the inferior ranks of animals; not only are they contented with the slowness of their pace and the wretchedness of their food, but they only very rarely seek help from their neighbours. It is not, however, uncommon to find some living among corals, which have even been designated coralligenous molluscs. There exists a group of Gasteropods, the Eulimæ, which lodge in certain Echinoderms, and in every respect deserve to be classed among messmates; it was a long time before the relation which exists between them and the animals which shelter them had been thoroughly appreciated. Dr. Gräffe found one species, the _Eulima brevicula_, on the _Archaster typicus_ of the Uvea Islands, in the Pacific Ocean. The molluscs, known by the name of _Stylifer_, have the same mode of life; they have been observed in the Asteriæ, the Ophiuræ, the Comatulæ, and even in the Holothuriæ; and as they inhabit the digestive cavity of these animals, it was believed that they frequented them as parasites. This was the opinion expressed first by d'Orbigny, and adopted by most naturalists. Professor Semper found some in the skin of a holothurian (_Stichopus variegatus_), which he considered incapable of nourishing themselves otherwise than at the expense of their host. However this may be, these molluscs, ranged alternately among the _Phasianellæ_, the _Turritellæ_, the _Cerithia_, the _Pyramidellæ_, the _Scalariæ_, 37 the _Rissoairia_, or in a distinct family, seem to belong rather to messmates than to parasites. We meet with Stylifers at the entrance of the mouth (Montacuta); more frequently they prefer, like the Fierasfers, to lodge themselves deeply in the digestive cavity in the midst of the _débris_ of the prey. The Melania (_M. Cambessedesii_, Risso), which Delle Chiaie found in the Bay of Naples, on the foot of some comatulæ, belongs probably to this group of molluscs.

Among the gasteropod molluscs which are not able to maintain themselves, we may mention another, a curious parasite, which instals itself in one of the rays of a star-fish, and whose presence is revealed by a swelling which is not produced in the other rays. This mollusc has received the name of _Stylina_.

The molluscs which are the most remarkable from the point of view from which we are now considering them, are the _Entoconchæ_; they live in Enchinoderms, and it was thought for a while that we could see in them an example of the transformation of one class into another. Some years since J. Müller found in a Synapta from the Adriatic, tubes with male and female organs, without any other apparatus, and in these tubes appeared eggs, whence this great physiologist saw molluscs proceed, with a helicoid shell, similar to that of a small natica; he gave them the name of _Entoconcha mirabilis_. Professor Semper has since discovered another species of these, which he has dedicated to the illustrious physiologist of Berlin, and which he found attached to the cloacal sac of the _Holothuria edulis_.

The true relation between these molluscs and the holothurians remains 38 to be discovered, and how the entoconchæ become at last simple sexual tubes. At present we must admit that it is the result of a retrogressive development like that of the peltogasters, which, like them, lose all the attributes of their class. They ought, perhaps, to be placed farther on, among parasites.

Some years since, some molluscs were observed which have compromised more or less the dignity of their class. Gräffe cites a species of the genus _Cypræa_, which one would certainly not expect to find in this category; it lives among the Viti Islands, in the compartments of the _Milithæa ochracea_. We have referred to it before. Naturalists have given the name of Melithæa to a very beautiful polyp which forms colonies of two or three metres in height. Mons. Steenstrup, with that perspicacity which discerns the most complex phenomena, has also described _Purpuræ_ which live as messmates with the Antipathes and the Madrepores. Quite recently, indeed, Mr. Stimpson has observed in the port of Charleston, a gasteropod mollusc, similar to a Planorbis (_Cochlioelepsis parasitus_) which lives as a messmate in the body of an annelid (_Ocoetes lupina_).

It is not the same with a mollusc called _Magilus_, which naturalists considered for a long time to be the calcareous tube of an annelid. All conchologists know the shell of the Magili, so valued by collectors. This gasteropod when young takes up its lodgings in the substance of a madrepore which grows more quickly than he, and in order not to die, stifled in this living wall, he constructs a calcareous tube similar to the shell, of which it appears to be the continuation, and which allows it to procure for itself water, air, 39 and food. The animal, protected by the madrepore, can do without its calcareous mantle, and only shows the end of the tube at the outside. It is this organ which sustains the struggle against the exuberant growth of the polyp, since it is by means of it that the mollusc obtains nourishment. The Magilus is like an oyster which is living in contact with a bank of mussels, with this difference, that the oyster almost always succumbs, while the magilus is always victorious in the struggle. We might also cite as well as the Magili, some _Vermeti_, certain _Crepidulæ_ and _Hipponices_, which struggle with the same success against those which pilot or receive them.

As there exist parasites which only depend on others during their youth, so there are messmates which are completely independent when fully grown. Jacobson, of Copenhagen, wrote, in or about 1830, a memoir to show that the young bivalves which are found in the external branchial processes of the Anadontæ are parasites, and he proposed for them the name of _Glochidium_. Blainville and Duméril were charged to make a report on this memoir, which the author had sent to the Académie des Sciences. But his opinion had not many supporters, and it is now thoroughly known that the young anodonts differ considerably in their early and their full-grown state. During their stay in the branchial tubes, each young animal carries a long cable which descends from the middle of the foot, and serves to attach the anodont to the body of a fish, and yet permits it to move to a certain distance.[1] In fact the young anodonts have, not like the other acephala, 40 vibratory wheels in order to move themselves; they are conveyed in this manner by their neighbours. There are also messmate acephala, as the _Modiolaria marmorata_, which lodge on the mantle of ascidians. Professor Semper found attached to the skin of a _Synapta similis_, a mollusc which possesses a peculiarity rare among these animals, that of carrying its shell in the interior and not on the outside.

There are few animals so infested with parasites as the Ascidians in general. Not only does their surface sometimes become a _microcosm_, as the name of one Mediterranean species indicates, but even in the substance of their testa lodge _Crenellæ_ and other molluscs and polyps, which choose by preference to place their dwelling there. There are also Annelids which hollow out galleries in their interior, Lernæans which establish themselves in their respiratory cavity, Nematodes, Pycnogonidæ, Ophiuræ, and many others besides. Mons. Alfred Giard has described several Amphipods and Isopods which establish themselves on Tunicates. One cannot say that there is always such a complete agreement between animals of such different kinds, for Mons. Alfred Giard gives examples of grave disagreements which he has seen break out, and which have caused the death of several among them.

Another association is that of a gasteropod with one of the acephala. In the environs of Caracas lives an Ampullaria (_Crocostoma_) which lodges in the umbilicus of its shell another mollusc, the only fluviatile species of those countries, called the _Sphaerium modioliforme_. We have every reason to suppose that the Sphaerium lives on good terms with the Ampullaria, since they are usually found associated.

The Bryozoaria, the animal mosses, establish themselves on all solid 41 bodies at the bottom of the sea, like true mosses on stones or on trees. One species, a _Membranipora_, is usually found on the common mussel. These animals are of small size, group themselves in colonies on the surface of shells and of polyparies, or even on crustaceans, and form by their union a fine kind of lace, the dazzling whiteness of which often comes out sharply on the varying and glittering colour of the shell. This is because each animal lodges in a cell which is not larger than the head of a pin, and all the cells of a colony are grouped together with the symmetrical regularity of the façade of a Gothic building.

Many Bryozoaria live in such a manner that it is impossible to say whether they are messmates, or have installed themselves by chance in a hiding-place for which they have no predilection. A charming bryozoon is developed in abundance on the carapace and the claws of the _Arcturus Baffini_, on the coast of Greenland, and propagates itself with extreme rapidity. On a single Arcturus we have found, scattered over its claws by the side of each other, Balani, Spirorbes, Sertulariæ, and vast colonies of Membranipora. One can see, merely by this example, the great zoological riches of the polar seas.

Certain annelids off the coasts of Normandy and Bretagne are the abodes of a bryozoary known under the name of _Pedicellina_, or _Loxosoma_. This interesting animal, which my fellow-labourer, Mons. Hesse, took for a Trematode, and whose drawings had led me into error, lives like others at liberty while young, and soon fixes itself to a Clymenian, in order to pass as a messmate the later period of its life. We have called it _Cyclatella annelidicola_, because of 42 its residence in a Clymenian annelid. Claparède and Keferstein have observed a species, the _Loxosoma singulare_, on a capitellian annelid, of the genus _Notomastus_, at St. Vaast-la-Hogue, on the coast of Normandy. After this, Claparède found another species, the _Loxosoma Kefersteinii_, in the bay of Naples, on an _Acamarchis_, a bryozoarian mollusc. Mons. Kowalewsky has observed in the Bay of Naples the _Loxosoma Napolitanum_.

We found some years ago the Pedicellinæ in so great abundance in the oyster beds of Ostend, that the baskets and other things floating on the water were literally covered with them. We have several times since endeavoured to procure them again, but it was in vain to search in the same places where they were formerly so abundant: we have not been able to discover a single one.

The class of worms includes not only parasites, it contains also, as we shall see, true messmates; we find some on crustaceans, on molluscs, on animals of their own class, on Echinoderms, and on Polyps.

One of the most curious of these worms is the _Myzostoma_, whose true nature has just been revealed by the excellent researches of Mons. Mecznikow. These myzostomes resemble trematode worms, but they have symmetrical appendages, and are covered with vibratory ciliæ. They live on the comatulæ, and run upon these echinoderms with remarkable rapidity. They have not hitherto been found elsewhere; they are evidently no more parasites than the last mentioned, and their place is among free messmates. Two great annelids are found, the one, the _Nereis bilineata_, by the side of Paguri in the same shell, the other, the _Nereis succinea_, according to Grube, in the tubes or galleries 43 of the Teredines. These dangerous acolytes introduce themselves furtively into the retreat of their host; and, always on the watch, they obtain at all times, and in every place, a certain prey, and a hiding-place from which they can take their share of their neighbour's goods. Another nereis, observed by Delle Chiaie, _Nereis tethycola_, lives in the cavities of a sponge, the _Tethya pyrifera_, which is visited by so many messmates and parasites, that it becomes a kind of hotel, where every one establishes himself at his ease. Risso also mentions a _Lysidice erythrocephala_ which lives in sponges.

In the same class is found an Amphinoma, a beautiful red-blooded worm, which proudly wears a plume of red branchiæ on its head, and which Fritz Müller observed on the coast of Brazil, begging assistance from a poor _Lepas anatifera_. Many Polynoës live upon other annelids; the _Harmothoë Malmgreni_ on the sheath of the _Choetopterus insignis_, the _Antinoe nobilis_ on the case of the _Terebella nebulosa_. Prof. Ray Lankester has lately communicated some observations on this subject to the Linnæan Society of London, and Dr. M'Intosh mentions some new species leading the same kind of life on the coast of Scotland.

Grube found at Trieste, in a star-fish (_Astropecten aurantiacus_), between its rows of suckers, a _Polynoë malleata_, with its stomach attached to the animal; and Delle Chiaie has lately observed on an asteria, a _Nereis squamosa_ by the side of a _Nereis flexuosa_. Mons. Grube thinks that the nereis of Delle Chiaie is no other than the _Polynoë malleata_. Lobsters are often covered with very small tubicular worms, which invade the whole carapace, and which, as true 44 messmates, give themselves up to the caprices of their host. These are a kind of _Spirorbis_, which, under the form of small spiral tubes, instal themselves, by preference, on the limbs, the antennæ, or the claws.

Mr. A. Agassiz has seen on the coast of the United States, a Beroë (_Mnemiopsis Leidyi_) which gives lodging in its interior to worms which somewhat resemble the Hirudinidæ, and which doubtless live there as messmates. Mr. A. Agassiz has remarked to me another example of commensalism. On the coast of the territory of Washington, as far as California, is found a worm of the genus _Lepidonotus_, which always lives near the mouth of a star-fish, the _Asteracanthion ochraceus_ of Brandt; sometimes as many as five are found together on a single individual, and are placed on different parts of the ambulacral rays. Mr. Pourtalis and Mr. Verril have observed annelids lodged in the polypidoms of the Stylaster.

There are few fish on which are not found _Caligi_, charming crustaceans which please the eye by their attenuated shape and their graceful movements. On these Caligi, which sometimes literally cover the skin of cod-fish coming from the north, we often find a curious trematode, the _Udonella_, which resembles one of the small hirudinidæ. Should this worm be placed among messmates? What is the part which it plays? We are persuaded that it is the same as that of the histriobdellæ under the tail of lobsters, that is to say, that it clears off the eggs of caligi which do not arrive at perfection, but perish in the course of their evolution.

Roussel de Vauzème has mentioned another worm, a nematode, to which 45 he has given the name of _Odontobius_, and which lives on the palatal membranes (the whalebones) of the southern whale. It is evidently a messmate. It can get nothing from the whalebones, but it snaps up on their passage in the interstices of the baleen, small animals of all kinds which swarm in these waters. When we open the _Pylidium girans_, we often find in the interior of its digestive cavity a larva, which was once thought to be descended from it, but instead of being allied to the Pylidium, this larva comes from a nemertian known by the name of _Alardus caudatus_. The young nemertian never abandons his host until it approaches the period of puberty, and then all the individuals living under the same conditions emancipate themselves at once, to pass the rest of their days free and roving like their mother.

Worms which have less freedom, like the Distomians, are sometimes both messmates and parasites. We find a remarkable example of this in the _Distomum ocreatum_ of the Baltic. According to the observations of Willemoes-Suhm, this trematode passes its cercarial life freely in the sea, and instead of encysting itself in the body of a neighbour, it attaches itself to a copepod crustacean, the whole of the inside of which it devours, in order to clothe itself afterwards with the carapace of its victim. It is under the cover of its prey that it passes into the herring, and completes its sexual evolution.

Mons. Ulianin has recently found another Distome (_Distomum ventricosum_) which passes its cercarial life in freedom in the bay of Sebastopol, and completes its evolution in the fishes of the Black Sea. J. Müller has long since found Cercaria living freely in the Mediterranean.

We ourselves, some years ago, while making some researches among the 46 Turbellaria, found among the eggs of some ordinary crabs of our coasts (_Carcinus mænas_), an interesting worm which we named _Polia involuta_, but which Prof. Kolliker appears to have known before, and designated by the name of _Nemertes carcinophilus_. It is not known whether it plays the same part as the Histriobdellæ and the Udonellæ. Delle Chiaie, as well as Prof. Frey and Prof. Leuckart, make mention of another nemertian which inhabits the _Ascidia mamillata_. Among the nemertians, we may allude to the _Anoplodium parasita_, which lives in the _Holothuria tubulosa_, and the _Anoplodium Schneiderii_, inhabiting the intestines of the _Stichopus variegatus_.

According to Mr. A. Agassiz, a species of Planarian (_Planaria angulata_, Mull.), lives as a free messmate on the lower surface of the Limulus, and prefers to establish itself near the base of the tail. Mons. Max Schultze recognized last year this same messmate on a limulus, which had died at Cologne in the large aquarium, and which had been sent to him for his anatomical studies. He showed at the congress of German naturalists at Wiesbaden, in 1873, the drawing which he had made of this animal, which he thought new to science. We may remark in passing, that he arrived, by means of his anatomical observations on Limuli, at the same result as did my son by his embryogenic observations, namely, that these supposed crustaceans are to be regarded as aquatic scorpions. Mr. Leidy also makes mention of Planarian parasites (_Bdellura_), with a sucker at the extremity of the body; and Mons. Giard noticed a blue one on the body of a Botryllus.

But of all the Turbellaria, the genus which appears to us the most 47 interesting is the Temnophila, which Gay first observed on crabs at Chili, and which Professor Semper afterwards found on the crabs of the Philippine Islands. Gay and Phillipi found colonies of these animals on the body, the claws, and more especially the abdomen, of the _OEglea_. This messmate resembles a trematode by its form and by its posterior sucker, but by its entire character, and especially by its sexual organs, it belongs to the _Turbellariæ_. Mons. Blanchard calls it _Temnophila Chilensis_. Professor Semper saw at the Philippine Islands these Temnophilæ on river crabs, at five thousand feet above the level of the sea.

The _Cydippe densa_, a charming polyp of the Gulf of Naples, lodges in its gastro-vascular apparatus larvæ of annelids, which may as well be considered parasites as messmates. We owe to Panceri the first observations on these worms, of which two genera, _Alciopina_ and _Rhynconerulla_, seem to live in the same manner in their youth. A naturalist, whose loss is profoundly deplored by the scientific world, Claparède, occupied himself with observations on these annelids during the last years of his life. It appears that these worms are so common in these polyps, that four have been found at once in the same animal.

The Spoon-worm, named by OErsted, _Sipunculus concharum_, ought doubtless to find its place here. An oligochete worm, _Hemidasys agaso_, from the Gulf of Naples, lives on the _Nereilepas caudata_, and Claparède did not think it unworthy of his attention. The surest means of finding it, says this philosopher, is to look for it on this annelid; and our much regretted fellow-labourer at Geneva did not 48 abandon this messmate before he had completely studied it. Let us remark in passing, that Professor Grube published in 1831, at Königsberg, a special work on the abodes of annelids in general.

Cases of commensalism among the Echinodermata are still more rare. These animals are sufficiently provided with organs, both with respect to their food and their skin, not to require the assistance of their neighbours. We cannot rank as a phenomenon of commensalism, the conduct of the young Comatulæ, which fasten themselves, as Mr. A. Agassiz informs me, to the basal cirrhi of the adult echinoderms, and there form a little colony of young Pentacrinites.

We only know one Ophiurus (_Ophiocnemis obscura_), which lives as a messmate on a comatula, and consequently seeks assistance from an animal of its own rank. Another kind of Ophiuride (_Asteromorpha lævis_, Lym.) fixes itself on a _Gorgonella Guadelupensis_ of Barbadoes. Everything induces us to suppose that we shall find more than one species of echinoderm, which will take its place among these when their mode of life has been studied with greater care. Professor Lütken has just proved this by quite recently making known another _Ophiothela_, which lives in the straits of Formosa, and seems to be the messmate of an Isidian polyp, known under the name of _Parisis loxa_. Another species (_Oph. mirabilis_) from Panama, infests certain Gorgoniæ and sponges; a third is found in the Fiji Islands on the _Melitodes virgata_; a fourth at the Isle of France on Gorgoniæ; and a fifth at Japan on the _Mopsella Japonica_. There is also another in the Pacific Ocean, but its companion is not known.

Professor Mobius, as well as Dr. F. Martens, has noticed a _Hemicuryale 49 pustulata_ on a polyp of Jamaica, known under the name of _Verrucella Guadelupensis_. This is a curious instance of mimicry.

The class of polyps includes several species which seek for assistance from others, and are classed among messmates. One of the most remarkable is the Gigantic Medusa, which can extend its arms downwards to a hundred and twenty feet, and bears the name of _Cyanea arctica_; the disc is seven feet and a half in diameter, and when the animal is on the surface of the water, the fringes, which surround the cavity at its mouth, occasionally afford lodging in the midst of them to a species of actinia, which lives there as messmate. Sometimes three, and even four or five, are found on a single Cyanæa. This also is an observation due to Mr. A. Agassiz, which he has published in his interesting work, "Sea-side Studies." Prof. Haeckel supposed that the _Geryoniæ_ produce _OEginidæ_ by means of buds; but it appears that the learned professor was mistaken as to the nature of these buds; that instead of being produced one from the other, they have, according to Steenstrup, a completely different genealogy, being only united by conditions of good-fellowship. They may be truly called messmates.

Mons. Lacaze-Duthiers, who went to the coast of Africa to study corals, met with a young polyp which requires the assistance of another polyp in its early condition. This animal, to which he has given the name of _Gerardia Lamarckii_, lives on one of the Gorgoniæ, which it invades and stifles, as the lianas strangle the tree over which they spread themselves. But these same Gerardiæ can also develop themselves on the eggs of the _Plagiostoma_, and are then 50 capable of living separately. In the substance of this polyp lives a crustacean, the nature of which Mons. Lacaze-Duthiers has not yet made known.

The superb sponge, _Euplectella aspergillum_, the elegant structure of which cannot be sufficiently admired, is, unlike the Alcyonium of the Dromia, rooted to the soil, but nevertheless gives shelter to three kinds of crustaceans: Pinnotheres, Palemonidæ, and Isopods. These supposed plants have been known for many years under the Spanish name of _Regadera_, or the English "Venus' Flower-basket;" they were first brought from Japan, and afterwards from the Moluccas, and more recently from the Philippine Islands. In almost all the individuals which Professor Semper was able to study in those parts, were found the same crustaceans. These _Euplectellæ_ have just been met with to the south-west of Cape St. Vincent, by Wyville Thomson, who has brought up some from a depth of 1090 fathoms, while on board the _Challenger_. This skilful professor has discovered another sponge to the north-west of Scotland, at a depth of 460 fathoms; it bears the name of _Holtenia Carpenteri_; and I have in my possession a fine specimen which I owe to his generosity, and keep as a _souvenir_ of the delightful hospitality which he extended to me at the Edinburgh meeting.

There are also sponges which construct a dwelling in the abode of their neighbour. We find, among others, a small sponge known under the name of _Clione_, which establishes itself in the substance of the shell of oysters, and hollows out galleries as the teredo does in wood. Mr. Albany Hancock found twelve species of Clione on a single 51 Tridacna. They are evidently not parasites, and I am not sure if their place is properly among messmates. The oyster, and more especially the _Ostrea hippopus_, lodges three or four different sorts in its shell. These Cliones possess siliceous spicules, by means of which they hollow out galleries in the substance of shells. Mr. Hancock has published a monograph of this genus, in which he recognizes twenty-four species collected from different shells, and two other species, which he refers to the genus _Thoasa_.

The cliones are real lodgers which lead us to the _Saxicavæ_, the _Pholades_, and the _Teredines_; they seek their lodging in rocks or in wood; these lead directly to the sea-urchins, which also hollow out lodgings in rocks, but without penetrating deeply. Professor Allman has just observed a very remarkable case of commensalism between a sponge and one of the tubulariæ. The crown of the tubularia is extended at the entrance of the canals of the sponge; and the association is so complete, that the Edinburgh professor imagined that he had before his eyes a true sponge with the arms of a tubularia.

In the lowest ranks of the animal scale, there are certain kinds of animalcules, which establish themselves on the bodies of obliging neighbours, and take advantage of their fins in order to swim at their expense. Thus we often find the bodies of certain crustaceans covered with a forest of vorticellæ and other infusoria. They cause themselves to be towed like cirrhipedes, but they do not change their toilet like them, so that it cannot be said that they put on the livery of servitude. The kind of life led by several of these animalculæ is as 52 yet little known.

Mons. Leydig has found in the stomach of the _Hydatina Senta_ a messmate which much resembles an Euglena, and still more the _Distigma tenax_, Ehr.

[1] I owe this observation to Dr. W. S. Kent, who showed me, in London, anodonts attached in this manner to sticklebacks.