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.

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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.

Fig. 74—Stages in the Life-history of Hæmocera danæ, One of the Monstrillidæ. (From Lankester's "Treatise on Zoology," after Malaquin.)

A, Free-swimming nauplius larva; B, embryo after penetrating into the body of the worm Salmacina; C, D, E, successive stages in the body of the host; F, free-swimming adult female. (All greatly enlarged, not to same scale.) a′, Antennule; br, brain; e, nauplius eye; f, swimming feet; g.s., hairs on which the eggs are carried; m, position of mouth; md, hooked mandible of nauplius; n, nerve cord; ov, mass of eggs carried by female; ovy, ovary; pr, absorptive processes.

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