4 BRITISH PARASITIC COPEPODA. 



forms so abnormal and grotesque that, as already 

 observed, it is difficult without a knowledge of their 

 life-histories to regard them as belonging to the 

 Crustacea. 



The young Copepod, no matter how abnormal it 

 may be when adult, begins life as a free-swimming 

 nauplius ; it is of an oval form, with a single median 

 eye and three pairs of limbs arranged round the 

 mouth ; there are no frontal appendages and no 

 proper mouth-organs. 



The young parasite passes through numerous 

 ecdyses and increases in size. The various append- 

 ages make their appearance in regular order, and 

 the animal gradually takes on the adult condition. 

 In the case of those parasites which retain their active 

 habits throughout life, the appendages become com- 

 pletely developed when the adult form is assumed. 

 On the other hand, the appendages of the parasites 

 that are more or less permanently attached to their 

 host are comparatively well developed early in life, 

 and then continue in a stationary condition or are 

 gradually suppressed. One finds on carefully dissect- 

 ing the head of an adult Lernasa from the gills of its 

 host that the four pairs of swimming-feet which were 

 developed at an early period of its life are present, 

 and retain the characters they had during the 

 cyclops stage, although they are not of any use to 

 the animal. The four pairs of feet are buried with 

 the head in the tissues, and if by any means the animal 

 could detach itself from its host, they obviously would 

 not enable it to swim through the water. In some 

 parasites certain appendages used as anchors, such 

 as the antennae and second maxillipeds, become 

 greatly developed and are conspicuous even to the 



