Birgus latroPLATE XXVIITHECOCONUTCRAB,Birgus latro. (MUCH REDUCED)View larger image
PLATE XXVII
THECOCONUTCRAB,Birgus latro. (MUCH REDUCED)
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The "Robber Crab" or "Coconut Crab" (Birguslatro—Plate XXVII.) also belongs to the family Cœnobitidæ, and has attracted much notice from its relatively gigantic size and its singular habits. Although resemblingCœnobitaclosely in essential structure,Birgusdiffers from it and from most other Hermit Crabs in not making use of a portable shelter, perhaps owing to the difficulty of obtaining one of suitable size. The necessary protection for the abdomen is obtained by a redevelopment of the shelly plates (terga) on the upper surface of the abdominal somites. The abdomen is carried doubled underneath the body to protect the soft under-surface, and the animal, when threatened, seeks a shelter for its vulnerable hinder part in the nearest hole or cranny. The swimmerets are absent in the male sex, and are present only on one side of the abdomen in the female. This unsymmetrical development of the appendages is interesting as indicating the derivation of the Robber Crab from ancestors adapted to living in the unsymmetrical shells of Gasteropod Molluscs. The last pair of abdominal appendages, which in other Hermit Crabs serve to hold the body in the shell, are here much reduced in size, and quite useless for that purpose. The carapace is very broad posteriorly, owing to the great development of the branchial cavities, which are much too capacious for the very small gills. As in the true Land Crabs, the lining membrane of the gill cavity is thick and spongy, and traversed bynumerous bloodvessels; but in this case its efficiency as a lung is added to by numerous tufted papillæ, which increase the surface exposed to the air.
As in other Hermit Crabs, the last two pairs of legs are shorter than the others, and they end in small chelæ. The last pair are very slender, and are usually carried folded up within the gill chambers, which they possibly serve to keep clear from foreign bodies. The penultimate pair of legs are stouter, and the two pairs in front of these are long walking legs. The chelipeds are very strong and are of unequal size. When attacked, the animal defends itself, not, as might have been expected, with its chelipeds, but with the first pair of walking legs, the sharp points of which form very efficient weapons.
The statement that the Robber Crab climbs lofty trees was first made by the Dutch naturalist Rumphius, in the beginning of the eighteenth century. Its accuracy has been often doubted or denied since then, and only finally put beyond dispute by a photograph taken on Christmas Island by Dr. Andrews, which shows one of these Crabs in the act of descending the trunk of a sago-palm. It seems not impossible that the habits of the animal may vary to some extent in different localities, and that where food is abundant on the ground the tree-climbing habit may be in abeyance. If this were so, it would explain the very definite statements made by some observers,thatBirgusdoesnotclimb trees.
In localities where coconut palms abound,Birgusfeeds largely on the nuts, tearing off the fibrous outer husk and breaking open the shell by hammering with its powerful claws at one of the "eye-holes." According to Darwin in his "Naturalist's Voyage," the pincers of the penultimate pair of legs are used for extracting the contents of the nut, but this observation does not seem to have been confirmed. In spite of its name of "Coconut Crab," however,Birgusby no means feeds exclusively on coconuts. On Christmas Island, where until recently there were no coconut palms, the Crabs are exceedingly abundant, and, according to Dr. Andrews, they "eat fruits, the pith of the sago-palm and the screw-pines, dead rats and other carrion, and any of their fellows that may have been injured.... They are excellent scavengers, and have a curious habit of often dragging their food long distances before attempting to eat it. I have seen a Crab laboriously pulling a bird's wing up the first inland cliff, half a mile or more from the camp whence it had stolen it."
Large specimens of the Robber Crab may be at least a foot in length of body when the abdomen is straightened out. Their great strength is illustrated by the fact, related by Darwin, that specimens placed in a strong biscuit-tin, of which the lid was secured by wire, escaped by turning down the edgeswith their claws, and in doing so actually punched holes quite through the tin.
The breeding habits and mode of development of the Robber Crab have often formed the subject of inquiry by naturalists, but it is only recently that Dr. Willey has been able to prove definitely that the female visits the sea for the purpose of hatching off the eggs, and that the young are hatched in the zoëa stage. The larvæ obtained by Dr. Willey have been described by Mr. Borradaile, who finds that, as was to be expected, they closely resemble those ofCœnobita. There appears, however, to be no such simultaneous migration of the Crabs towards the sea as has been described in the case of the Gecarcinidæ. The statement, quoted by Darwin, thatBirgusvisits the sea every night for the purpose of moistening its branchiæ, cannot be universally applicable, since the Crabs are often found, as on Christmas Island, at distances from the sea which put a nightly journey to it out of the question.
Of all Crustacea, the most completely adapted to terrestrial life are the Land Isopods, or Woodlice, which may be found in every garden. It is true that most species are found in damp places, although some that inhabit the sandy deserts of Asia and Africa must be content with a very slight degree of humidity; and in no case is their dependence on moisture greater than, for instance, that of many Insects and Arachnids which are regarded as typicallyterrestrial animals. Since there is reason to believe that the Woodlice have been derived from marine ancestors—they show no special affinities to the fresh-water Isopoda, likeAsellus—it is interesting to find that the most primitive forms, which have departed least from the general Isopod type, are commonly found on or near the seashore. The "Sea-slater,"Ligia oceanica(Fig. 63,), which is abundant in rocky places on our own coast, is one of the most primitive forms. It has a broad, flattened, greenish-brown body, about an inch long, and it runs quickly, creeping into narrow crevices of the rocks, so that it is not easy to catch. The antennules, as in the other land Isopods, are very minute, but the antennæ are long, and have, besides the fivesegments which form the "peduncle," a "flagellum" of about twelve short segments. The uropods or tail appendages are long, each with two slender, pointed branches. On the under-side of the abdomen can be seen the five pairs of pleopods, each with two plate-like branches attached to a very short peduncle. As in most aquatic Isopods, the plates of the pleopods are soft and thin, and appear adapted to act as gills, although the outer plate of each pair is somewhat stiffer than the inner. The Sea-slater is generally found just above high-water mark, probably always within reach of the salt spray, and it is said sometimes to enter the water of rock-pools.
The Sea-slaterFig. 63—The Sea-slater(Ligia oceanica).About Twice Natural Size.(After Sars.)View larger image
Fig. 63—The Sea-slater(Ligia oceanica).About Twice Natural Size.(After Sars.)
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In almost every garden there may be found, under flower-pots and the like, a Woodlouse, about two-thirds of an inch long, of a brown colour, with yellowish blotches arranged in a row on each side of the back. This isOniscus asellus, a species widely distributed in Europe and North America. It has the antennæ shorter than inLigia, and the flagellum is composed of only three segments. The uropods are quite short. The endopodites of the pleopods are membranous gill-plates, which serve for respiration in the moist air in which these animals generally live. The exopodites are stiff plates which cover and protect the delicate endopodites; it is probable that they also aid in respiration, for they contain a system of minute channels, filled with air, where the cuticle is separated from the underlying cells. Asthese channels are nowhere open to the outside, the air must find its way in by diffusion through the cuticle.
Structure of the Breathing Organs of Porcellio scaberFig. 64—Structure of the Breathing Organs ofPorcellio scaber. (From Lankester's "Treatise on Zoology," after Stoller.)A, Exopodite of first pleopod, showing the tuft of air-tubes ("pseudo-tracheæ"), seen through the transparent cuticle; B, vertical section through same; C, part of section more highly magnified.art, Point of attachment of exopodite to peduncle;c, cuticle;gr, grooved area of cuticle;hy, hypodermis, or layer of cells under the cuticle;n, nucleus of hypodermis cell of air-tube;o, external opening;tr, air-tubesView larger image
Fig. 64—Structure of the Breathing Organs ofPorcellio scaber. (From Lankester's "Treatise on Zoology," after Stoller.)
A, Exopodite of first pleopod, showing the tuft of air-tubes ("pseudo-tracheæ"), seen through the transparent cuticle; B, vertical section through same; C, part of section more highly magnified.art, Point of attachment of exopodite to peduncle;c, cuticle;gr, grooved area of cuticle;hy, hypodermis, or layer of cells under the cuticle;n, nucleus of hypodermis cell of air-tube;o, external opening;tr, air-tubes
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Armadillidium vulgareFig. 65—Armadillidium vulgare.× 2-½. (After Sars.)
Fig. 65—Armadillidium vulgare.× 2-½. (After Sars.)
Even more abundant thanOniscus asellus, and often found together with it, isPorcellio scaber(seeFig. 20, p. 51). It is usually of a dark bluish-grey, but occasionally it is irregularly mottled with a lighter colour. The flagellum of the antenna has only two segments. The most interesting difference fromOniscus, however, is found in the pleopods. If the under-side of the living animal be examined with a pocket lens, a white spot will be seen on eachexopodite of the first two pairs of pleopods. When the structure of the pleopods is investigated by means of microscopic sections (Fig. 64), it is found that the white spots are tufts of fine branching tubes radiating into the interior of the exopodite from a slit-like opening on the outer edge. These tubes arise by an in-pushing of the integument, and they are lined throughout by a delicate continuation of the external cuticle. During life they are filled with air, and they serve to aerate the blood circulating in the interior of the appendage.
Another Woodlouse common in England isArmadillidium vulgare(Fig. 65), a little slaty-grey species with a very convex body, which rolls itself into a ball when touched. Like the last-mentioned species, it has two segments in the flagellum of its short antennæ, and it has tufted air-tubes in the exopodites of the first two pairs of pleopods. It is often mistaken for an animal of widely different structure, which it superficially resembles—the Pill Millipede (Glomeris marginata). The latter, however, may easily be recognized by having either seventeen or nineteen pairs of walking legs (instead of seven pairs), set close together in the middle line of thebody, and by lacking the plate-like pleopods. The resemblance between the two animals can hardly be regarded as a case of "mimicry," since there is no reason to believe that either benefits by its likeness to the other. As in so many other cases of "convergent resemblance" between animals of different structure, it does not seem possible to get beyond the vague suggestion that a similarity in habits may have led, in some way that we do not understand, to a similarity in appearance.
The presence of air-tubes in the pleopods of many Woodlice raises some questions which are of importance with reference to the classification of the Arthropoda as a whole. The Six-legged Insects, most Spiders and many of their allies, the Centipedes and Millipedes, and the worm-likePeripatus, all breathe air by means of fine tubes which penetrate throughout the body, and bring the air into close contact with the tissues. These tubes, which are known as "tracheæ," arise as ingrowths of the outer layer of the embryo, and are lined by a delicate continuation of the external cuticle. It has been held by some zoologists that so peculiar a system of breathing organs must indicate a common descent of the animals that possess them, and accordingly it has been proposed to separate the Insects, Arachnids, Myriopods, andPeripatus, as a group, Tracheata, from the Crustacea and some other Arthropods which have no tracheæ. The air-tubes of the Woodlice,however, are precisely like tracheæ in structure and function, and only differ from the tracheæ of the other groups in the fact that they are confined to the appendages, and do not penetrate into the body. Since the Woodlice are a small and highly specialized branch of the Crustacea, we can hardly suppose that they derive their tracheæ from any ancestral type which they had in common with the widely different Arachnids, for example; and if tracheæ have been evolved independently in these two groups, there seems no reason why those of the Insects may not have arisen independently of either. This is only one example out of many which go to show that, in attempting to reconstruct the genealogy, or phylogeny, as it is called, of the animal kingdom, we must constantly admit the possibility of "convergent evolution."
Although Woodlice are very common animals, comparatively little is known of their habits. They seem to live chiefly on vegetable food, and sometimes damage seedlings and tender plants in gardens and greenhouses, but occasionally they are carnivorous, and even cannibalistic, in their habits. A few species live as "guests" in ants' nests, and one of these, the little blind whitePlatyarthrus hoffmannseggii, is common in many localities in this country. Why the ants tolerate their presence we do not know, for they do not seem to render any service to their hosts, as do the plant-lice and some other insectsthat are kept by the ants for the sake of the secretions which they yield.
The Woodlice, like some other Isopoda, have a peculiar method of moulting. Instead of the whole exoskeleton being cast off at one time, as in other Crustacea, that of the hinder half of the body is moulted first, and it is only after two or three days, when the new cuticle has hardened, that the exoskeleton of the anterior half follows. As a result of this arrangement, it occasionally happens that specimens are found with the fore part of the body differing in colour from the hind part, owing to the one having been moulted more recently than the other.
Woodlice occur in most regions of the globe, and one of the most remarkable features of their geographical distribution is the extremely wide range of certain species. This is probably due, at least in many cases, to their accidental transport by human agency. Thus,Porcellio scaber, so common in this country, is also found in great abundance in New Zealand; but Professor Chilton notes that it is usually found near buildings, and only rarely in the native bush, so that there can be no doubt that it has been introduced by artificial means.
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 foodfrom 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 ofConchoderma, 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.Tubicinellais 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 gillchambers, profiting no doubt by the respiratory currents and the food particles they carry.
Coronula diademaPLATE XXVIIIGROUP OF BARNACLES,Coronula diadema,ON THE SKIN OF A WHALE.JAPAN.(REDUCED)View larger image
PLATE XXVIII
GROUP OF BARNACLES,Coronula diadema,ON THE SKIN OF A WHALE.JAPAN.(REDUCED)
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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 Cœlentera, 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 actuallyprey 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 branchesof 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."
Two Branches of a CoralFig. 66—Two Branches of a Coral(Seriatopora)showing "Galls" inhabited by the CrabHapalocarcinus marsupialis.On the Right the Female Crab, extracted from the Gall and further enlargedView larger image
Fig. 66—Two Branches of a Coral(Seriatopora)showing "Galls" inhabited by the CrabHapalocarcinus marsupialis.On the Right the Female Crab, extracted from the Gall and further enlarged
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The Medusæ, or Jellyfishes, like other Cœlentera, 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 genusHyperia(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.
Hyperia galbaFig. 67—Hyperia galba,Female.Enlarged.(After Sars.)View larger image
Fig. 67—Hyperia galba,Female.Enlarged.(After Sars.)
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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 ofthe 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 CrabParapagurus pilosimanus(seePlate XVI.) is always foundlodged 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 byPaguropsis 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 byEupagurus bernhardus(seePlate VII.) often contain one of the bristle-footedworms (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 CrabDromia(seePlate 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.
Melia tessellataFig. 68—A, The CrabMelia tessellataclinging to a Branch of Coral, and carrying in Each Claw a Living Sea-anemone; B, One of the Claws further enlarged to show the Way in which the Anemone is held.(After Borradaile.)View larger image
Fig. 68—A, The CrabMelia tessellataclinging to a Branch of Coral, and carrying in Each Claw a Living Sea-anemone; B, One of the Claws further enlarged to show the Way in which the Anemone is held.(After Borradaile.)
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One of the strangest habits is that of certain little tropical Crabs, of whichMelia 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 theywouldbe of little use by themselves for attack ordefence; 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.
Pinnotheres pisumFig. 69—The Common Pea Crab (Pinnotheres pisum),Female.Natural Size.View larger image
Fig. 69—The Common Pea Crab (Pinnotheres pisum),Female.Natural Size.
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From remote antiquity it has been known that a little Crab (Fig. 69) is frequently found living withinthe shells of bivalve Molluscs, such as Oysters, Mussels, and especially the large mussel-likePinna, which is common in the Mediterranean. Ancient writers regarded this as a case of association for mutual advantage, believing that thePinnothereswarned thePinnaof 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 ofPinnotheresand 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 extentdegenerate 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.
Cirolana borealisFig. 70—Cirolana borealis.About Twice Natural Size.(After Sars.)View larger image
Fig. 70—Cirolana borealis.About Twice Natural Size.(After Sars.)
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While most of the Pinnotheridæ live in bivalve Molluscs, some species are associated with other invertebrate animals.Pinnaxodes chilensisis 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 thePinnain place of the Crab. This isPontonia 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 ofCirolana, which havepowerful biting jaws. OfC. 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 littleEurydice 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 ofÆgaand 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 thepower 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 ofÆgadilates 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 speciesÆga 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.
PLATE XXIXCymothoa oestrumCymothoa œstrum,AN ISOPOD PARASITE OF FISH(SLIGHTLY ENLARGED)View larger imageSacculina carciniSacculina carciniATTACHED UNDER THE ABDOMEN OF A COMMON SHORE-CRAB(REDUCED)View larger image
PLATE XXIX
Cymothoa oestrumCymothoa œstrum,AN ISOPOD PARASITE OF FISH(SLIGHTLY ENLARGED)View larger image
Cymothoa œstrum,AN ISOPOD PARASITE OF FISH(SLIGHTLY ENLARGED)
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Sacculina carciniSacculina carciniATTACHED UNDER THE ABDOMEN OF A COMMON SHORE-CRAB(REDUCED)View larger image
Sacculina carciniATTACHED UNDER THE ABDOMEN OF A COMMON SHORE-CRAB(REDUCED)
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The most completely parasitic members of the Cymothoidæ are found in the subfamily Cymothoinæ, including the typical genusCymothoa(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 thissubfamily. 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 genusSpirontocaris, isBopyroides hippolytes, represented inFig. 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 coveredin 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.
Effects of Bopyroides hippolytesFig. 71—A, Front Part of Body of a Prawn(Spirontocaris polaris),from Above, showing on the Right Side a Swelling of the Carapace caused by the Presence of the ParasiteBopyroides hippolytesin the Gill Chamber; B, the Female Parasite extracted and further enlarged; C, the Male Parasite on Same Scale as the Female.(After Sars.)View larger image
Fig. 71—A, Front Part of Body of a Prawn(Spirontocaris polaris),from Above, showing on the Right Side a Swelling of the Carapace caused by the Presence of the ParasiteBopyroides hippolytesin the Gill Chamber; B, the Female Parasite extracted and further enlarged; C, the Male Parasite on Same Scale as the Female.(After Sars.)
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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 theBopyrusjust 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 completelyknown. 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. InBopyrusand many other genera, however, there is no evidence that the males ever develop into females.
A Fish-louseFig. 72—A Fish-louse(Caligus rapax),Female. × 5. (After Wilson.)View larger image
Fig. 72—A Fish-louse(Caligus rapax),Female. × 5. (After Wilson.)
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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 whichhave suctorial mouth parts, and lead a semi-parasitic existence; but the only completely parasitic forms are the Whale-lice, forming the family Cyamidæ (seeFig. 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 numerousspecies of the family Caligidæ, many of which, belonging to the generaCaligus(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 inCaligusthe 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 ofChalimus, the parasite is unable to detach itself from its host; but later, inmany species, it re-acquires the power of swimming, and specimens ofCaligus, for instance, are commonly found free in tow-net gatherings.