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 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.

Fig. 73—Stages of Development of Lernæa branchialis. F is slightly, the Other Figures greatly, enlarged. (After A. Scott.)