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. [Illustration: FIG. 72--A FISH-LOUSE (_Caligus rapax_), FEMALE. × 5. (After Wilson.)] 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. [Illustration: FIG. 73--STAGES OF DEVELOPMENT OF _Lernæa branchialis_. F IS SLIGHTLY, THE OTHER FIGURES GREATLY, ENLARGED. (After A. Scott.) A, Nauplius, just hatched; B, young female taken from gills of Flounder; C, free-swimming stage of female, after leaving Flounder; D, free-swimming male; E, female just after settling on gills of Whiting; F, fully-developed female.] 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.