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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.

[CHAPTER XI]

CRUSTACEA IN RELATION TO MAN

The Crustacea come into relation with human life in the most obvious and direct way in the case of those species that are used for food. The number of species so used in various parts of the world is very large, almost the only necessary condition being that the species shall be sufficiently large and abundant to make it worth while to fish for it.

As most of the larger Crustacea belong to the Decapoda, it is this order that supplies practically all the edible species, almost the only exceptions being a few Barnacles which are eaten in various parts of the world. Thus the sessile Barnacle Balanus psittacus, found on the coasts of Chili, and growing to a length of 9 inches by 2 or 3 inches diameter, is, according to statements quoted by Darwin, "universally esteemed as a delicious article of food," and the pedunculate Pollicipes cornucopia is used for food on the coasts of Brittany and Spain.