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. [Illustration: 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.]