CHAPTER X

CRUSTACEA AS PARASITES AND MESSMATES

The life of every animal is in more or less intimate relation with that of all the living creatures which surround it. Some serve for its food, or supply it with shelter or foothold; others prey upon it, or compete with it for the necessaries of life; and others, again, influence it for good or evil in countless ways more subtle than these, but equally important. There are some associations of a closer and more enduring nature, to which the names of Symbiosis, Commensalism, and Parasitism, are applied, and it is with examples of these that the present chapter is concerned.

The term Symbiosis is strictly applied to an intimate physiological partnership, such as we find in some of the lower animals and plants, and in this sense there are no truly symbiotic Crustacea. The word, however, is sometimes used, in its literal sense of a "living together," to embrace all cases of animals living together for mutual advantage. Commensalism means, literally, "sitting at the same table," and ought to be applied only to cases where two or more animals, living together as "messmates," partake of the same food; but it is sometimes used more loosely to include instances where one of the animals does not actually share in the food-supply of the other. Parasitism, again, implies that the parasite lives permanently at the expense of its host, by sucking its juices or otherwise, and in this case also there are innumerable degrees and varieties of dependence, which defy inclusion in a strictly logical scheme of classification. Even such typical parasites as Tape-worms, for example, might strictly be regarded as commensals, sharing in the host's food only after it has entered the alimentary canal. Finally, in all these kinds of interrelation, we find cases where the association is temporary, intermittent, or almost accidental, and where there are no perceptible adaptations of structure directed to its maintenance in either of the partners. From these we may trace a series of gradations leading to cases where the associated organisms are never found apart, and where the structure of both is profoundly modified in adaptation to the particular form of association.

Perhaps the simplest form of association between two animals is found where one utilizes the other as a means of transport. The little Gulf-weed Crab, previously mentioned, is very often found clinging to the carapace or skin of large marine Turtles. It is not a parasite, since it can hardly derive any food from the Turtle itself; neither is it a commensal, for there is no evidence that it shares in the Turtle's meals. It probably takes to a Turtle, when it can find one, as giving it a wider range of operations than is afforded by its usual drift-log or tuft of sargasso-weed. A somewhat similar case is afforded by some of the Barnacles that are found on the skin of Whales. The species of _Conchoderma_, for instance, are often found on certain Whales, but they may also occur on inanimate floating objects. Other Whale-infesting Cirripedes, however, are specially adapted to their habitat, and never occur elsewhere. For example, _Coronula_ (Plate XXVIII.) is a genus of sessile Barnacles in which the shell is elaborately folded, forming a series of chambers into which prolongations of the Whale's epidermis grow, securely fixing the shell. _Tubicinella_ is even more effectively protected against dislodgment, for its shell is sunk in the thickness of the Whale's skin, with only the opening exposed. Other genera of sessile Barnacles (_Chelonobia_, etc.) are found adhering to the shell of Turtles. The increased food-supply made available by the host's movements through the water is probably the chief advantage that the Barnacles gain in such cases. This is indicated by the fact that certain small stalked Barnacles (_Dichelaspis_, etc.), found on large Crabs and Lobsters in tropical seas, generally cluster on the mouth parts of their hosts, near the entrances to, or even within, the gill chambers, profiting no doubt by the respiratory currents and the food particles they carry.

A great variety of Crustacea find shelter and defence in association with Sponges, Corals, and other more or less sedentary animals. Sponges are not eaten by many marine animals, the needle-like spicules which often form their skeleton no doubt helping to render them distasteful, and many small Crustacea, Amphipods, Isopods, Prawns, etc., profit by their immunity from attack, and take up their abode in the internal channels and cavities of the Sponge. The beautiful siliceous Sponge known as "Venus's Flower-basket" (_Euplectella_) very often contains imprisoned within it specimens of a delicate little Prawn (_Spongicola venusta_) or of an Isopod (_Æga spongiophila_). As these Crustacea share with the Sponge the food particles drawn in by the currents of water passing through the pores in its walls, they are in the strict sense commensals.

The Corals and various other animal organisms commonly known as "Zoophytes," forming together with the Jellyfishes the group Coelentera, are very effectively protected against the attacks of most predatory animals by the possession of "stinging cells," and this protection is shared by many other animals which shelter among them. Thus, the branching Coral stocks which grow in great luxuriance on tropical coasts support a rich and varied assemblage of animals, some of which may actually prey upon the Coral polypes, but all of which profit by the fact that few enemies venture to pursue them in their retreats. Innumerable prawn-like animals of the Alpheidæ and other families, and many kinds of Crabs, are found among living Corals. The Crabs of the family Trapeziidæ are especially characteristic of such habitats, and their thin, flat bodies seem to be adapted to slip into slits and crannies of the Coral blocks. The most highly specialized of all Coral Crabs, however, are the species of the family Hapalocarcinidæ, which modify in various ways the growth of the corals on which they live. In some of the more delicately branched kinds of Coral there may sometimes be found hollow bulbous growths, each of which contains imprisoned within it a little Crab--_Hapalocarcinus marsupialis_ (Fig. 66). It seems that the female Crab (the habits of the male are not definitely known) settles down among the branches of the Coral, and that the irritation of its presence causes the branches to grow up and surround it, coalescing with each other to form a kind of cage, and ultimately leaving only one or two small openings. Through these openings water can enter to enable the Crab to breathe, and no doubt food particles find their way in, but it is not possible for the Crab to leave its prison. The production of these abnormal growths of the Coral is closely analogous to the formation of "galls" on plants as a result of the irritation set up by the presence of insect larvæ or other parasites, and it is not inappropriate, therefore, to speak of them as "Coral galls."

The Medusæ, or Jellyfishes, like other Coelentera, are provided with poisonous stinging cells, which, in the larger species of our own seas, are powerful enough to cause discomfort to bathers who come in contact with them. The protection thus afforded is no doubt of advantage to the little globular Amphipods of the genus _Hyperia_ (Fig. 67), which are almost always to be found sheltering under the bells of the larger Medusæ. In what way the Amphipods escape injury from the stinging cells of their host is not known.

In all the cases mentioned, the advantages of the partnership seem to be all on one side, but there are numerous instances in which both partners seem to reap some benefit. A species of Hermit Crab very common in moderately deep water on many parts of the British coasts, _Eupagurus prideauxi_, is always found to have a Sea-anemone (_Adamsia palliata_) attached to the shell which it carries. The Anemone has a broad base which is wrapped round the shell, the mouth, surrounded by the tentacles, being on the under-side next the opening of the shell. There seems no reason to doubt that the presence of the Anemone does afford some degree of protection to the Hermit, and that, on the other hand, the Anemone benefits by being carried about, and shares in the crumbs from the Hermit's meals. It is stated that, when the Hermit removes to a new shell, it detaches the Anemone from the old shell with its pincers and places it in position on the new one. It appears, however, that it is not always necessary for the Hermit to remove to a larger shell as it grows, for the enveloping Anemone, as it increases in size, extends beyond the mouth of the shell, and so enlarges the shelter. Further, the Anemone in course of time dissolves the shell almost entirely away, and the Hermit is enveloped only by the soft fleshy mantle which it forms.

In a similar way the deep-sea Hermit Crab _Parapagurus pilosimanus_ (see Plate XVI.) is always found lodged in a fleshy mass formed by a colony of Sea-anemones (_Epizoanthus_), within which, when it is cut open, may be found the remains of the shell which the Hermit first inhabited. A further development of the same habit is given by _Paguropsis typica_, found in deep water in Indian seas, which does not inhabit a shell at any time, but carries a fleshy blanket formed by a colony of Anemones.

In dredging off the British coasts, we often find smooth rounded lumps of a Sponge (_Suberites ficus_), generally yellowish-grey in colour, having a round opening in which the claws of a small Hermit Crab (_Eupagurus cuanensis_) may be seen. On cutting open the Sponge, the body of the Hermit is seen to be lodged in a spiral cavity, and at the apex may be found the remains of a shell that has been corroded away by the Sponge which settled on and replaced it. Other species of Hermit Crabs constantly have their shells covered with a horny crust formed by Hydroid zoophytes (_Hydractinia_, etc.), and in this case also the extension of the Hydroid colony beyond the lip of the shell relieves the Hermit from the necessity of so frequently changing to a larger shell as it grows.

A number of other animals are found associated with Hermit Crabs, without, as far as we can see, rendering any service in return for the house-room. The Whelk-shells inhabited by _Eupagurus bernhardus_ (see Plate VII.) often contain one of the bristle-footed worms (_Nereilepas fucata_), which may sometimes be observed to protrude its head from the shell when the Crab is feeding, and to snatch away fragments of the prey from the very jaws of its host. It is thus, in the strict sense of the word, a commensal. Species of Copepods, Amphipods, Porcelain Crabs, and even a Mysid, have been found sharing the lodging of Hermit Crabs in a similar way, and in addition there are various parasites, presently to be mentioned, found on the Crabs themselves, so that each Crab forms the centre of a whole community of widely diverse organisms all more or less directly dependent on it.

A habit similar to those of some Hermit Crabs is that of the Crab _Dromia_ (see Plate IX.), mentioned in a previous chapter, which carries, as a cloak, a mass of living sponge, holding it in position by means of the last two pairs of legs. Even the "masking" habit of the Spider Crabs, already described (p. 96), may be regarded as a kind of symbiosis, since the sponges, zoophytes, etc., which grow on the Crabs no doubt benefit by being carried about in return for the protection they give.

One of the strangest habits is that of certain little tropical Crabs, of which _Melia tessellata_ (Fig. 68) is the best known, which carry in each claw a living Sea-anemone and use it as a weapon. The claws or chelipeds are in this case of small size, so that they would be of little use by themselves for attack or defence; but the fingers are provided with recurved teeth, enabling them to take a firm hold of the slippery body of the Anemone. Particles of food caught by the tentacles of the Anemone are removed and eaten by the Crab, which uses for the purpose the long walking legs of the first pair. The same limbs are also used in the process of detaching the Anemones from the stone on which they may be growing. The Anemones do not appear to suffer from the rough treatment to which they are subjected, but whether they can reap any benefit from the partnership is very doubtful.

From remote antiquity it has been known that a little Crab (Fig. 69) is frequently found living within the shells of bivalve Molluscs, such as Oysters, Mussels, and especially the large mussel-like _Pinna_, which is common in the Mediterranean. Ancient writers regarded this as a case of association for mutual advantage, believing that the _Pinnotheres_ warned the _Pinna_ of the approach of enemies or of the entrance of prey between its gaping valves. It is even stated that the Pinna and Crab were depicted in Egyptian hieroglyphics to symbolize the dependence of a man on his friends.

As a matter of fact, however, there is no reason to believe that the Molluscs which harbour species of _Pinnotheres_ and allied genera benefit in any way by the presence of the Crabs. The latter probably feed, as their hosts do, on particles brought in by the current of water entering the mantle cavity. They are therefore strictly "commensals," though it is usual, and perhaps equally correct, to speak of them as "parasites." The case is, indeed, an example of the difficulty of defining these two terms. At all events, the Pinnotherid Crabs show one of the characteristics of parasites in being to some extent degenerate in their structure. The carapace and the rest of the exoskeleton, no longer needed for protection, have become soft and membranous, and the eyes and antennules, the chief organs of sense, are very minute. As in many parasites, also, the eggs produced by the female are very numerous, and the abdomen is very broad and deeply hollowed for their reception.

While most of the Pinnotheridæ live in bivalve Molluscs, some species are associated with other invertebrate animals. _Pinnaxodes chilensis_ is found in a species of Sea-urchin (_Strongylocentrotus gibbosus_) on the coast of Chili. On opening the shell of the Urchin, the Crab is found enclosed in a thin-walled bag formed by enlargement of the terminal part of the host's intestine.

It did not escape the notice of Aristotle that a little Shrimp sometimes occurred in the _Pinna_ in place of the Crab. This is _Pontonia custos_, and other species of the same and allied genera have similar habits.

The order Isopoda includes a very large number of parasitic species. The extensive family Cymothoidæ presents a whole series of gradations in habits and structure between actively swimming predatory species and others which in the adult state are permanently fixed to their host, usually a fish, and are incapable of movement. At one end of the series are the species of _Cirolana_, which have powerful biting jaws. Of _C. borealis_ (Fig. 70), Mr. Stebbing remarks that "it is a good swimmer, tenacious of life, a savage devourer of fish, and not to be held in the human hand with impunity." The species is not uncommon in British seas, and numerous individuals will sometimes attack a Cod or other large fish, perhaps after it has been caught on a hook, and gnaw their way into its body, so that when brought to the surface the fish consists of little more than skin and bone.

The little _Eurydice achatus_, belonging to the same subfamily, Cirolaninæ, is commonly taken in the tow-net in sandy bays on our own coasts. It is said sometimes to attack bathers, and to "nip most unpleasantly."

More definitely parasitic are the species of _Æga_ and allied genera, which have piercing and suctorial mouth parts, and suck the blood of fish. They are usually found adhering closely to the skin of their victim by means of the strong hooked claws of the anterior pairs of legs; but they have not lost the power of locomotion, and, as females bearing eggs are never taken on fish, it would appear that they drop off after gorging themselves with blood, and probably seek a retreat at the bottom of the sea, where they may hatch their young in safety. The digestive canal of _Æga_ dilates into a large bag, which becomes distended with a semi-solid mass of blood. This mass, when extracted and dried, is the "Peter's stone" of old Icelandic folklore, to which magical and medicinal virtues were attributed. The species _Æga spongiophila_, already mentioned, differs in its habits from all the other species of the genus, since it lives, not on fish, but in the interior of a sponge.

The most completely parasitic members of the Cymothoidæ are found in the subfamily Cymothoinæ, including the typical genus _Cymothoa_ (Plate XXIX.) and many others. The adult animals are found clinging to the skin of fishes, the legs being provided with strong hook-like claws that give them a very firm hold. Some species, especially common on Flying-fishes, cling to the tongue of the fish, and almost prevent it from closing its mouth. When young, the Cymothoinæ swim freely, and the shape of the body is not unlike that of the Cirolaninæ; but after they have settled on a host the body often becomes distorted and unsymmetrical. A still more remarkable change occurs in the reproductive organs in some, if not in all members of this subfamily. Each individual, when it first attaches itself to a host, presents the characters of the male sex. Later it becomes a female, develops a brood-pouch, and produces eggs. The animals are, in fact, hermaphrodite; but it is to be noted that the hermaphroditism is of a different kind from that presented by the Cirripedia, since the organs of the two sexes are successively, not simultaneously, developed. Where, as in this case, the male phase comes first in the life-history of the individual, the condition is known as "protandrous" hermaphroditism.

Another large group of parasitic Isopods is the suborder Epicaridea, all the species of which are parasitic on other Crustacea. It is not uncommon to find specimens of the common Prawn (_Leander serratus_) which have a large swelling on one side of the carapace. If the lower edge of the carapace be raised, it will be seen that this swelling is due to the presence in the gill cavity of an Isopod parasite (_Bopyrus squillarum_). A closely similar form, found on Prawns of the genus _Spirontocaris_, is _Bopyroides hippolytes_, represented in Fig. 71. Other allied species are found on Hermit Crabs and other Decapods. When extracted, the parasite is seen to have a flat and curiously distorted body, with extremely short legs ending in hooked claws. The under-side is generally occupied by a relatively enormous mass of eggs, which is only partly covered in by the small brood-plates. The mouth parts form a short piercing beak with which the parasite sucks the blood of its host. On the under-side of the abdomen may usually be found the minute male, attached, like a secondary parasite, to the body of the female.

The species of Epicaridea are very numerous, and they infest Crustacea belonging to nearly all the chief groups of the class, a few even being parasitic on other Epicaridea. Many of them differ greatly from the _Bopyrus_ just described, and in some cases it would be impossible to guess from the structure of the adult animals that they were Isopoda, or even Crustacea at all. The life-history is not yet completely known. When hatched from the egg, the free-swimming larvæ have a short and broad body, and, as in other Isopod larvæ, have only six instead of seven pairs of legs. A later larval stage, just before attachment to the final host, has a long narrow body and the full number of legs. It has lately been shown, however, that, in all probability, between these two free-swimming stages there intervenes a stage in which the larvæ is temporarily parasitic on certain Copepoda. Further, some of the Epicaridea, like the Cymothoinæ described above, are protandrous hermaphrodites, developing the male organs when in the last larval stage, and passing into the female phase after they have become attached to the host. In _Bopyrus_ and many other genera, however, there is no evidence that the males ever develop into females.

Some of the most remarkable Epicaridea are those belonging to the family Entoniscidæ, which are parasitic on Crabs. In these the parasite penetrates from the gill chamber into the interior of the body of the host, remaining enveloped, however, by a delicate membrane which grows in with it from the wall of the gill chamber. The body is distorted in an extraordinary fashion, so that at first sight it seems impossible to trace any resemblance to the form of a typical Isopod.

Among the Amphipoda there are a few species belonging to various families of the Gammaridea which have suctorial mouth parts, and lead a semi-parasitic existence; but the only completely parasitic forms are the Whale-lice, forming the family Cyamidæ (see Fig. 23, p. 55) in the suborder Caprellidea. Although differing greatly in the broad, flattened shape of the body from the slender, thread-like Caprellidæ, they closely resemble them in structure, particularly in having the abdomen reduced to a mere knob. The fourth and fifth pairs of thoracic limbs have disappeared, although the gills corresponding to them are very large; and the last three pairs of legs have long curved claws which enable the Whale-louse to cling firmly to the skin of its host. The mouth parts are adapted for biting, not for sucking blood, and the animals seem to live by gnawing the skin of the Whales. In one respect the Whale-lice are unique among Crustacean parasites: they have not the power of swimming at any period of their life-history. The young settle down near their parents, and masses of many hundred individuals of all sizes are found clinging close together on the skin of the host.

No group of Crustacea exhibits more numerous or more varied examples of parasitism than the Copepoda. Every grade of transition between a free predatory habit of life and the most complete dependence upon a host may be traced in various families of the subclass. Only a few examples can be mentioned here.

The commonest "fish-lice" are the numerous species of the family Caligidæ, many of which, belonging to the genera _Caligus_ (Fig. 72), _Lepeophthirus_, etc., are found on marine fishes on our own coasts. In these the body is broad and flat, but in many of them the resemblance, even in general form, to the free-living Copepoda is easily traceable. The maxillipeds form powerful hooked claws, by means of which the animals cling to the skin of the fish they infest, and in _Caligus_ the basal segments of the antennules have a pair of suckers which aid in adhesion. The mouth parts are adapted for piercing, and are enclosed in a suctorial proboscis.

When the young Caligid, after passing through the free-swimming larval stages, first becomes attached to a fish, it adheres by means of a thread-like process issuing from the front of the head, and formed by the secretion of a gland. At this stage, formerly described as an independent species under the generic name of _Chalimus_, the parasite is unable to detach itself from its host; but later, in many species, it re-acquires the power of swimming, and specimens of _Caligus_, for instance, are commonly found free in tow-net gatherings.

On the gills of Cod, Haddock, and other common fish, we often find a red worm-like parasite, _Lernæa branchialis_ (Fig. 73, F), which at first sight seems to bear no sort of resemblance to a Crustacean. The soft body is curiously doubled up, and is attached to the host by a narrow neck; while dissection will reveal a small head buried in the flesh of the fish's gills, and having three branched outgrowths, which penetrate into the surrounding tissues and make the attachment of the parasite more secure. Near the hinder end of the body are two coiled threads, which are the egg-masses. The reduced mouth parts and the microscopic vestiges of the swimming feet may be detected on and near the head, but apart from these it would be hard to find any characters to show that the animal is a Crustacean.

The life-history of _Lernæa_ is very remarkable. The young are hatched in the nauplius stage (Fig. 73, A), and after passing through some further free-swimming stages they become parasitic on a fish. Curiously enough, however, they choose a very different host from that on which the adults are found, for at this stage (Fig. 73, B) they attach themselves to the gills of one of the Flat-fishes (Pleuronectidæ), such as the Flounder, Plaice, etc., attachment being effected by a frontal cement gland similar to that of the larval Caligidæ, already mentioned. The animal is now without the power of swimming, its appendages becoming reduced to stumps and losing their setæ. After passing some time in this condition, the larva again acquires the power of swimming, and leaves its host. Both sexes become mature in this free-swimming stage (Fig. 73, C, D), and impregnation is effected. The males die without developing further, but the females seek a second host, a fish of the family Gadidæ, such as the Cod, Haddock, etc., and, settling on the gills, become metamorphosed (Fig. 73, E) into the adult form described above.

Within the gill cavities of the strange-looking fish known as the Angler or Fishing-frog (_Lophius piscatorius_) there may often be found specimens of another parasitic Copepod, _Chondracanthus gibbosus_. It has a soft, unsegmented body about half an inch long, provided with numerous blunt lobes which give it a very irregular shape. On the under-side, near the front, are forked lobes representing two pairs of the swimming feet. At the hinder end are usually attached a pair of long thread-like egg-masses. Just at the point where the egg-masses are attached, close inspection of the under-side of the body will reveal a very minute maggot-like object. This is a male individual, which is attached, like a secondary parasite, to the body of the enormously larger female.

In all the cases mentioned, the animal is parasitic in the final state of its existence--at least in the female sex--but there are a few Copepoda which are free-swimming, both when young and when adult, but parasitic in the intermediate stages. Among the Copepoda taken by the tow-net in British seas, there may sometimes be found species of the family Monstrillidæ (Fig. 74, F), which are remarkable for having no appendages between the antennules and the first pair of swimming feet. They have no trace of jaws, and only a minute vestige of a mouth-opening; while internally there is no food-canal, so that the animals are incapable of taking nourishment. Their development was for long a mystery, but it is now known that the greater part of their life is passed as internal parasites in certain bristle-footed worms (Polychæta). The young are hatched as nauplius larvæ (Fig. 74, A) without mouth or food-canal, but capable of swimming, and having the third pair of appendages (mandibles) furnished with strong hooks, by means of which they fasten on to the worm which is to serve as their host. The nauplius bores through the skin of the worm, casting its cuticle and losing all its appendages in the process, and making its way into one of the bloodvessels in the form of a little oval mass of cells (Fig. 74, B), within which no organs except the degenerating nauplius eye can be detected. It later becomes enclosed in a delicate cuticle, and from one end two long finger-like processes grow out, which are believed to have the function of absorbing nourishment from the blood of the host (Fig. 74, C, D). Within the cuticle the organs of the adult animal are gradually differentiated (Fig. 74, E), and when fully formed it bores its way through the tissues of its host by means of rows of hook-like spines surrounding the pointed posterior end of the sac. On reaching the surface the enclosing membrane bursts, and the adult animal is set free.

Of all Crustacean parasites, however, perhaps the most remarkable in their structure and life-history are the Cirripedes of the order Rhizocephala. It is not uncommon on the British coasts to find specimens of the common Shore Crab or other Crabs which carry under the abdomen an oval fleshy body. This is the Rhizocephalan _Sacculina carcini_ (Plate XXIX.), and it would hardly be possible to guess, from its appearance or structure, that it was a Cirripede or a Crustacean at all. It is attached to the under-side of the Crab's abdomen by a short stalk, and in the middle of its opposite surface is a small opening which leads into a cavity separating the outer "mantle" from the body of the animal. Very often this mantle cavity will be found to be full of eggs enclosed in sausage-shaped packets. At the point where the short stalk enters the abdomen of the Crab, it gives off an immense system of fine branching roots, which penetrate throughout the body of the Crab, and even into its legs and other appendages. By means of these roots the _Sacculina_ absorbs nourishment from the body-fluids of its host. Like most Cirripedes, _Sacculina_ is hermaphrodite, and the body within the mantle cavity contains only the reproductive organs of the two sexes and a small nerve ganglion representing the whole of the nervous system. There is no mouth, no food-canal, and no trace of appendages. Another Rhizocephalan, _Peltogaster_, is not uncommonly found attached to the abdomen of Hermit Crabs. Although the nauplius larva of _Sacculina_ was described, and its resemblance to that of the Cirripedia pointed out, as long ago as 1836, by that acute observer, J. Vaughan Thompson, it is only recently that the full life-history has been made known by the researches of Professor Delage and Mr. Geoffrey Smith. The nauplius larva (Fig. 75, A) resembles that of the normal Cirripedes, especially in the shape of the dorsal shield, which is drawn out on either side in front into a pair of fronto-lateral horns. It has, however, no mouth, and the food-canal is quite absent. As in the normal Cirripedes, the nauplius is followed by a _cypris_ stage (Fig. 75, B), also mouthless, and it is in this form that the _Sacculina_ seeks the Crab on which it is to become parasitic. It would be almost impossible for the _cypris_ larva to settle on that part of the Crab where the adult _Sacculina_ is afterwards to appear, since the Crab usually has its abdomen closely pressed against the under-side of its thorax. The larva therefore attaches itself on some exposed part of the Crab, often on one of the legs, clinging to a hair by means of its antennules. It bores through the cuticle at the base of the hair, and the contents of its body pass into the interior of the Crab as a little mass of cells, the empty _cypris_ shell being cast off. This mass of cells, which constitutes the embryo _Sacculina_, is carried about by the blood-currents of the Crab till it reaches the under-side of the intestine, where it becomes attached. It now begins to send out roots (Fig. 76), and as it grows the central mass travels backwards along the intestine of the Crab till it reaches the place where the adult parasite is to emerge. As the mass increases in size, and the organs of the _Sacculina_ become differentiated within it, its presence causes the living tissues between it and the external cuticle to degenerate, so that when the Crab moults an opening is left through which the body of the parasite protrudes. Owing, no doubt, to the drain on its system due to the presence of the _Sacculina_, the Crab ceases to grow, and it does not moult again as long as the parasite remains alive.

In addition to this arrest of growth, _Sacculina_ produces in its hosts other changes, which affect chiefly the reproductive organs and the structures associated therewith. Crabs of either sex infected with _Sacculina_ are incapable of breeding; the genital gland (ovary or testis) is found on dissection to be shrivelled up, and the external characters indicative of sex become strangely modified. The changes have been most fully studied in the case of a kind of Spider Crab common at Naples--_Inachus mauritanicus_. In this species it is found that females infected with _Sacculina_ show no conspicuous external modification, except that the abdominal appendages, which in the normal females serve for the attachment of the eggs, are greatly reduced in size. Infected males, however, may assume to a greater or less degree the characters proper to the female sex. Some males show little change, except that the chelipeds remain small and flattened, as in the females and non-breeding males. Other specimens have, in addition, the abdomen much broader than in normal males, and sometimes as broad as in the females. Finally, some males develop on the abdomen, in addition to the rod-like appendages on the first and second somites, characteristic of the male sex, two-branched appendages on the next three somites, as in the females; these individuals are, in fact, so completely intermediate in character between the two sexes that it is only by dissection that it is possible to recognize them as modified males.

An indication of the way in which the degenerate Rhizocephala have been derived from normal Cirripedes is given by a peculiar species of pedunculate Barnacle, _Anelasma squalicola_, which lives attached to Sharks and Dogfish in the North Sea. In _Anelasma_ the peduncle becomes deeply buried in the flesh of the Shark, and its surface is covered with short branching, root-like filaments. As in the case of the Rhizocephala, these roots appear to absorb nutriment from the host, and, although _Anelasma_ possesses a food-canal and mouth, the cirri are reduced in size and devoid of hairs, so that they cannot be used for obtaining food as in ordinary Barnacles.