140 ANIMAL PAEASITES AND MESSMATES. 



modified ; the male often preserves all the attributes of 

 his sex and of his youth, while the female changes 

 entirely her appearance and her mode of motion, espe- 

 cially at the approach of the period when the interest of 

 the species prevails over that of the individual. 



We can nowhere find more graceful and regular forms 

 during the whole of their early youth than those of 

 many of these parasites ; we can never see more ungrace- 

 ful, we might almost say more comical, attitudes than 

 those of the greater part of these creatures when full 

 grown. One might take them for some misshapen 

 excrescence, or some scrap of wasted flesh on the body of 

 their host. A certain number of insects are found which 

 lead this singular kind of life, but this is more especially 

 the case among the crustaceans, particularly the copepod 

 crustaceans. Among all these we find the most absurd 

 recurrent forms ; in fact these animals instead of carrying 

 on their evolution, like the caterpillar which becomes a 

 butterfly, retrograde rather than advance, and acquire 

 an appearance and character which prevent us from 

 recognizing their origin. Many of these are at present 

 known, whose graceful form is so completely changed, 

 that without referring to the study of their embryo state, 

 one could not tell to what class they belong. Nothing 

 remains of their organs except the sexual apparatus and 

 a shapeless skin. These curious parasites live also on 

 the surface of bodies, and sometimes in the cavity of the 

 mouth ; but in fishes they are most frequently found in 

 the branchial membranes. They look like natural setons, 

 and it is not impossible that they sometimes fulfil the 

 same functions. 



We will first examine some insects, then certain 



