PARASITES AND MESSMATES 227 



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 dissec- 

 tion 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 Lern&a 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 

 (Pleuronectidae), such as the Flounder, Plaice, etc., 

 attachment being effected by a frontal cement gland 

 similar to that of the larval Caligidae, already 

 mentioned. The animal is now without the power 

 of swimming, its appendages becoming reduced to 

 stumps and losing their setae. After passing some 



