CHAPTER IV. 68

MUTUALISTS.

In this chapter we bring together animals which live on each other, without being either parasites or messmates; many of them are towed along by others; some render each other mutual services, others again take advantage of some assistance which their companions can give them; some afford each other an asylum, and some are found which have sympathetic bonds which always draw them together. They are usually confounded with parasites or messmates.

Many insects shelter themselves in the fur of the mammalia, or in the down of birds, and remove from the hair and the feathers the pellicle and epidermal _débris_ which encumber them. At the same time they minister to the outward appearance of their host, and are of great utility to him in a hygienic point of view.

Those which live in the water have other guardians: instead of insects, we find a number of crustaceans which establish themselves on fishes, and if there are no scales of the epidermis which annoy them, there are mucosities which are incessantly renewed in order to protect the skin from the continual action of the water.

We find many on the surface of the scales, and others which conceal 69 themselves at the bottom of mucous canals. We have brought together only a few examples, and there are certain others which are mentioned elsewhere, but which ought more properly to be placed here.

The insects long known under the name of _Ricini_, and to which many other appellations have been given, deserve to figure in the first rank in this group. They have always perplexed entomologists, who seem to consider them as parasites allied to acaridæ and lice. It has, however, been long known that they have no trunk to suck with, and that they have two small scaly teeth, which rather serve for the purpose of biting. A long time since, the examination of their stomach proved that they contain only morsels of skin instead of blood. This has induced many entomologists to place them in the same order as grasshoppers, that of Orthoptera.

Lyonet has given figures of several of those which he studied with the care which he so well knew how to employ in his anatomical investigations; and in 1818 Nitzsch, a professor at Göttingen, had brought together so great a number of them, that it required several days to examine his collection; he began the publication of his catalogue, but has not had time to finish it. Several other entomologists and anatomists have since taken up the subject.

We owe the description of several hundred species to Mr. Denny. Mons. F. Rudow has lately made known a great number of species which he has collected from the skins of birds coming from Japan, Australia, 70 Africa, and the two Americas.

Professor Grube, of Breslau, has published the description of the insects and acaridæ found during the travels of Middendorf in Siberia. These descriptions relate especially to the Philopteræ of birds, the Pediculinæ of the mammalia, a flea of the _Mustela Siberica_, and an acarus of the _Lemmus_. Quite recently, an American naturalist, Mr. Packard, who has undertaken the study of so many different subjects, has published in the "American Naturalist" the description, accompanied by an engraving, of the _Menopon picicola_, found on the _Picoides Arcticus_ from the lower Geyser basin, Wyoming territory, also of the _Goniodes Merriamanus_, the _Tetrao Richardsoni_, and the _Goniodes mephitidis_, found on a _Mephitis_ from Fire-Hole Basin, Wyoming territory; of the _Nirmus buteonivorus_, from a _Buteo Swainsonii_; and of _Docophorus Syrnii_, from _Syrnium nebulosum_.

A great number of these insects live between the feathers of birds, and can be more easily observed, since they detach themselves after the death of their host. They are easily found on the skins of birds prepared for museums. These ticks form a family under the name of _Riciniæ_, and this family is divided into two parts, the _Liotheidæ_ and the _Philopteridæ_.

Among the many generic divisions, one of the most interesting has received the name of _Trichodectes_; it contains twenty species, one of which lives on the dog, another on the cat, another on the ox; in a word, we discover a distinct species on each of the domestic mammals. It has been said that the _phthiriasis_ of the cat is occasioned by 71 the abundance of ricini. The trichodectes of the dog has lately attracted the especial notice of naturalists, and that from the following circumstances.

There is no tape-worm more common in the dog than the _Tænia cucumerina_. But whence comes it? How is it introduced? This had been an enigma for many years, at the time when I dissected some dogs infested with _Tænia serrata_, in the Museum of Natural History at Paris. Together with the _Tænia serrata_, the number and age of which I knew beforehand, since I had myself _planted_ them, there were found in the intestines of one of the dogs some individuals of the _Tænia cucumerina_. My dogs had taken nothing but milk, and _cysticerci pisiformes_. Were there cysticerci of different kinds in the peritoneum of the rabbit? The veil is now withdrawn. We have just said that the dog harbours a tick known under the name of Trichodectes, and in this trichodectes lodges the Scolex, we might even say the larva of the _Tænia cucumerina_. Dogs, especially young ones, lick their hair continually, and it is by this operation that the young tænia is introduced. It is by a similar process that the horse introduces the eggs of the OEstrus which are hatched in its stomach.

Many of these ticks live abundantly in birds, and multiply rapidly. The _Liothe pallidum_ lives on the cock, the _Liothe stramineum_ on the turkey, the _Philopterus falciformis_ on the peacock, the _Philopterus claviformis_ on the pigeon. It is to be observed that every bird can nourish many different kinds. Fig. 2 represents the tick which infests the sea-eagle, called Pygarg.

The name Arguli has been given to some crustaceans which resemble the caligi in size and in manner of life, and which principally frequent fresh-water fishes. The _Argulus foliaceus_ is the name of the species which has been known for the longest time, and which is most extensively found. It is to be seen on our pikes, carps, sticklebacks, 74 and on the greater part of our river fish. Mr. Thorell, in his monograph, mentions twelve species of Arguli proper, and four species of which he composed the genus _Gyropeltis_. Four are found in Europe, two of which are on salt-water, and two on fresh-water fish.

Quite recently, Professor Leydig has made known another species living on the _Phoxinus levis_. Arguli are met with on the fishes of Africa, the Indies, and North and South America. Like the caligi, these animals spontaneously abandon one host, to go and attend to the toilet of another.

Another animal, which has been taken for a Lernæan, deserves to take its place by the side of the Caligi, at least on account of its manner of life. We refer to that singular being which Leydig discovered in 1850 in Italy, while studying the mucous canal of a _Corvina_, at Cagliari, and to which he gave the name of _Sphoerosoma_. To judge by the plate and by some details, this _Sphoerosoma_, the name of which ought to be changed to _Leydigia_, belongs, if we mistake not, to the same group as the Histriobdellæ. We are persuaded that the first opportunity will confirm the correctness of this alliance, by the study of its embryonic form. If we had not been able to examine into all the development of the Histriobdellæ, more than one naturalist would have considered them Lernæans, as happened at the congress of German naturalists at Carlsruhe.

If we see many of these crustaceans live a joyous life while young, there are others which seem to practise economy, and to emancipate themselves when they have grown old. Mons. Hesse and Mr. Spence Bate a few years since revealed the secrets of their existence.

Naturalists had recognized some crustaceans under the name of 75 _Ancei_, and others under the name of _Pranizæ_, living together upon fishes, but with very different organs for fishing and swimming. M. Hesse, curious to know the manner of life of the Pranizæ, made observations on them in a small aquarium, and he perceived that the parts of the mouth were all at once transformed into formidable mandibles, which caused them to resemble Ancei. As it had often occurred with respect to other groups, that the same crustacean at different periods of its evolution had been taken for different animals, the naturalist of Brest had some suspicion as to their identity, and soon ascertained by direct observation that he had not been mistaken. The Pranizæ become Ancei, and live upon fishes under their first form, like caligi and arguli. Nothing can be seen which is more curious than these crustaceans, which ride on the back or the sides of fishes, and assume there every possible attitude.

The Pranizæ fix themselves in the mouth and in the gills as well as on the skin. Some are found on sharks as well as on osseous fishes. They fear neither heat nor light, and do very well under damp sea-weed while waiting for the return of the tide. They run and swim with the same facility. When in the condition of Ancei, they lose their agility, and, under this form, all denotes their sedentary habits. They appear to live in holes, at the bottom of which they defend themselves with their powerful mandibles. It has been observed that fecundation is accomplished, as in the _Axolotls_, before the evolution is complete, but that the eggs are not laid until the animal assumes the form of Anceus.

We may here remark that the change of appearance takes place only 76 among the females; the males preserve their dress and their liberty. Some naturalists assert that we must not accept the metamorphosis of either sex as an established fact, except for the purpose of arrangement. All, however, tends to show that Mons. Hesse has fairly interpreted facts; but it appears to us probable that the whole of the history of these strange crustaceans is not fully known.

Fishermen have long since known whale-lice, the _Cyami_ of naturalists, of which we have already made mention while speaking of free messmates. They live at liberty on the skin of their host, and multiply with extreme rapidity. These Cyami have a regular form, but completely different from the others, and have given (like the Ricini and the afore-mentioned crustaceans), great trouble to systematic zoologists. The place which they ought to occupy is far from being definitely fixed. At all events they may be considered as a shorter kind of Caprellæ.

As each whale has cirrhipedes which are peculiar to itself, so each has its own cyami. Professor Lütken, of Copenhagen, has made known ten or twelve species, all found on cetacea, in the two hemispheres. The supposed Cyamus, represented by Dr. Monedero as living on the Biscayan whale, is a Pycnogonon.

The Anilocræ and the Nerocilæ, like the Cyami and other genera, establish themselves on the back of a fish which is a good swimmer. Jealous of their liberty, they preserve their oars and their fins, in order to change their convoy, when the desire seizes on them, and do not imitate the Bopyrians, which instal themselves on the narrow 77 branchial cavity of some decapod crustacean, and as soon as they have entered, throw off all their travelling baggage; in fact, there is no other means for them to gain admission; their lot is identified with that of their host; they can no longer live without him. The female only, it is true, thus renounces her liberty; she sacrifices herself, as usual, for her family, while the male, far from giving himself up, preserves his defensive arms, his claws, and his liberty.

The crustaceans called Caprellæ are perhaps not so independent as they appear to be; it is not impossible that their place may be among the crustaceans now under our consideration. They are often found, together with the Tanaïs, on the bodies of cetaceans and chelonians, on plagiostomous fishes, or in the midst of colonies of Sertulariæ. They also establish themselves on buoys when they are well covered with animal life; and we have discovered them in prodigious numbers on a piece of cable which had lain at the bottom of the sea, and the whole surface of which was covered with animals of every kind.

We may here mention the Pycnogonons, the Saphyrinæ, the Peltidiæ, and the Hersiliæ; these crustaceans often crawl over the skins of their congeners, but without ever renouncing their independence; and they are all more or less occupied with the toilet of their neighbours.

We shall place in a second section some animals which have been usually classed among parasites, rather because of their living upon their neighbours than on account of their mode of life. If it is necessary in menageries to have keepers to cleanse the animals themselves, it is as requisite to have others to keep the cages 78 clean, and to remove dung and filth. Many animals perform this office. The rectum of frogs is always literally full of _Opalinæ_ which swarm in this cavity, like ants in their ant-hill, and doubtless live on the contents of the intestine.

These Opalinæ are true infusoria, which do not wait till the fecal matters are decomposed, and till the waters are corrupted by their presence; they prevent accidents which might arise, and interfere in time to purify the water from these excretions. There have been found hitherto in the rectum of frogs, and in the different annelids, the Pachydrili, the Clitelides, the Lumbriculi, and the Enchytrei. We have also seen them in the Planaria and the Nemertians. There is no sight more curious for those who are commencing microscopical studies, than the examination of the contents of the rectum of these Batrachians. Van Leeuwenhoeck knew, two hundred years ago, those animalculæ, to which Bloch at a later period gave the name of _Chaos intestinalis_. There are also some Rotatoria, the _Albertiæ_ for example, which ought to have a place here, and which Dujardin has described and named. They live in the intestines of the Lumbrici and of snails, and in the larvæ of Ephemerides.

Dujardin first pointed out the _Albertia vermiculus_; since then Mons. Schultze has made known the Albertia of the _Näis littoralis_, and Radkewitz has recognized in the small worm of our gardens the _Enchytreus vermicularis_. Long since, Siebold correctly stated that these animals are not parasites, since they do not live at the expense of their host.

There is a worm in the Philippine Islands, as Professor Semper has 79 informed me, which lodges in the intestines of a fish, with its head usually projecting outwards, and which watches the crustaceans attracted by the excreta of its host; but although it chooses the intestine of its neighbour as a place of shelter, it is not a parasite.

Fishermen affirm, and the examination of the animal's stomach confirms their assertion, that the _Cyclopterus lumpus_ feeds on nothing but the excreta of other fishes. Indeed, it is not possible to count the number of intestinal worms known by the name of _Scolex_, which are found in the contents of the stomach and the intestines. Besides this, we have long known the peculiarities of some insects which cannot live except on the dung of certain animals; and there is an example of one of these insects, found in a fossil state, which anticipated the discovery of the remains of an extinct mammal before unknown in that district. The larvæ of the fly _Scatophaga stercoraria_ live only on excrementary matter.

There are also nematode worms which exist under these conditions, and which develop and propagate their species in the intestines as if in the midst of damp earth. The small eel-like creatures so abundant in cow-dung propagate in it; they are not parasites, and are allied to those of which we speak in this chapter.

Besides those attendants which busy themselves about the cleanliness of other animals, we find some whose duties are less extensive, and whose cares are more limited. Many animals produce a greater number of eggs than they can bring to perfection, and those which are decomposed for want of fecundation, or which die in the course of evolution, 80 are under the care of an especial attendant, employed to make away from time to time with the addled eggs, or the embryos that have failed to come to maturity.

In this manner lobsters give lodgings in the midst of their eggs to a worm, which we at first took for a Serpula, and which, after a complete examination, turns out to be one of the Hirudinidæ: we have given it the name of _Histriobdella_. It is as singular in its movements as in its conformation, and its manner of living approaches that of the _Pontobdellæ_ of the rays, of which we shall speak subsequently. We announced this discovery a few years ago in the following terms:--

It is known that lobsters, as well as crabs and the greater part of the crustacea, carry their eggs under the abdomen, and that these eggs remain suspended there till the embryos are hatched. In the midst of them lives an animal of extreme agility, which is perhaps the most extraordinary being which has been subjected to the eyes of a zoologist. It may be said, without exaggeration, that it is a biped, or even quadruped, worm. Let us imagine a clown from the circus, with his limbs as far dislocated as possible, we might even say entirely deprived of bones, displaying tricks of strength and activity on a heap of monster cannon balls, which he struggles to surmount; placing one foot formed like an air-bladder on one ball, the other foot on another, alternately balancing and extending his body, folding his limbs on each other, or bending his body upwards like a caterpillar of the geometridæ, and we shall then have but an imperfect idea of all the attitudes which it assumes, and which it varies incessantly.

Its rank and its affinities would have given rise to long discussions 81 if we had not made known at the same time its evolution and anatomical structure.

It is neither a parasite nor a messmate; it does not live at the expense of the lobster, but on one of the productions of these crustaceans, much in the same manner as do the Caligi and the Arguli. The lobster gives him a berth, and the passenger feeds himself at the expense of the cargo; that is to say, he eats the eggs and the embryos which die, and the decomposition of which might be fatal to his host and his progeny. These Histriobdellæ have the same duty to perform as vultures and jackals, which clear the plains of carcases. That which causes us to suppose that such is their appropriate office, is that they have an apparatus for the purpose of sucking eggs, and that we have not found in their digestive canal any remains which resemble any true organism. We find the feces, rolled up as balls, placed after each other in their intestines.

The crustaceans also feed other Hirudinidæ. Mons. Leydig has noticed a _Myzobdella_ on the _Lupa diacantha_. The fresh-water crab, common in all the rivers of Europe, nourishes two, the _Astacobdella roeselii_, which lives under the abdomen, or about the eyes, and the _Astacobdella Abildgardi_ which especially frequents the branchiæ. Two astacobdellæ on the same crab doubtless play different parts. We should almost venture to assert, _à priori_, that the species in the gills lives as a parasite on the blood of its host, whilst the other, lodged under the abdomen, plays the same part as the histriobdella of the lobster.

We often find among the eggs of the ordinary crab of our coasts 82 (_Cancer moenas_) a nemertian which probably performs the same office. He is lodged while young in a kind of firm sheath attached to the abdominal processes. We have been able easily to study the first phases of its evolution. We have given it the name of _Polia involuta_.

This nemertian had been observed at Messina, and described before by Kölliker under the name of _Nemertes carcinophilus_, and it has just been described and figured anew by Mr. M'Intosh, in a monograph of British annelids published by the Ray Society.

The sturgeon seems to give lodging in its eggs to a polyp which plays the same part. In fact, Mons. Owsjannikoff, at the congress of Russian naturalists at Kiew, described an animal, _Accipenser ruthenus_, which lives in the eggs of the sterlet. Some eggs placed in water for a few hours at first show tentacles on the outside, then a whole colony, and each part consists of four individuals, which have a common digestive cavity, resembling somewhat a hydra divided longitudinally in four. Each has six tentacles, two of which are terminated by transparent corpuscles, perhaps nematocysts; the digestive cavity extends into the arms, as in the hydra; the mouth is not between the tentacles, but at the opposite pole. They are not all lodged within the eggs; some are found outside, according to the observations of Mons. Koch. Does not this animal fulfil in the egg of the sterlet, the same office as the histriobdella in the egg of the lobster?

The eggs of some insects are attacked by very little ichneumons, the _Proctotrupidæ_; they empty them, and then instal themselves in the shell. Mons. Fabre has mentioned, in his memoir on the habits of 83 the _Meloë_, a worm found in an egg.

M. Barthelemy has studied a nematode worm (_Ascaroides limacis_) which inhabits as a parasite the egg of the grey snail; is this not the ordinary worm of the snail which has introduced itself into the eggs?

Many animals establish themselves on their neighbours, not to obtain any advantage from them, except to profit by their fins; they are not themselves sufficiently adapted to rapid motion, so they seize a good courser, mount on his back, and ask from him only a resting-place and no provisions. But it is often very difficult to say where commensalism ends and mutualism begins; the cirrhipedes, for example, establish themselves on a piece of floating wood, or on the bottom of a vessel; on a block of stone, or on one of the piles of a groin; on an immovable animal as well as on a good swimmer.

Some fourteen years ago, Jacobson of Copenhagen wrote an interesting essay, to show that the young bivalves that are found in the branchiæ of anodonts at a certain period of the year are parasitical animals, for which he proposed a new name. But these supposed parasites are only young anodonts, which by the help of a very long cable, which proceeds from their foot like a byssus, attach themselves to their mother, or to a fish which will carry them to a distance.

We see full-grown acephalous molluscs, as mussels and pinnæ, still keep these cables, under the name of byssus, during their whole life. There are also among distomians, worms which though they are hermaphrodite, couple two and two, and have this additional peculiarity, that while one increases rapidly the other becomes 84 atrophied.

An Egyptian distome, which lives in man, gives an instance of this peculiarity, as well as the _D. filicolle_, which inhabits a fish (_Brama Raii_). The caligi which live on the skin of fishes are, when young, fastened by a cord which comes from the anterior edge of their carapace: while quite little, they put themselves under the protection of a kind neighbour, and allow themselves to be led by him.

The new tubularia, which we have dedicated to our learned colleague Dumortier, often fixes itself on the carapace of ordinary crabs, and causes itself to be conveyed like the Echeneis; the tubulary observed by Gwyn Jeffreys, close by the eye of the _Rossia papillifera_, a cephalopod mollusc, perhaps belongs to the same species.

Every colony of campanulariæ or sertulariæ lodges a crowd of messmates and mutualists; and there are a great number of crustaceans and polyps of all sizes which serve as an abode for infusoria of every kind. Some establish themselves on the carapace or on the swimming appendages, as in a carriage; others on one of the gills, which renders their mode of life more easy, and the danger less great. An amphipod very extensively spread over our sea-coasts, the _Gammarus marinus_, usually has its appendages covered with _Vayinicola crystallina_.