148 ANIMAL PARASITES AND MESSMATES. 



in development that Cuvier placed them by the side of. 

 the helminths. These creatures possess as soon as 

 they are born, all the attributes of their class, and wear 

 the dress of free crustaceans ; as they approach mature 

 age, they choose a neighbour, instal themselves as con- 

 veniently as possible in one of his organs, and get rid of 

 all their apparatus for fishing and hunting. The sexes 

 are usually separated, and as the female is specially 

 devoted to the cares of her progeny, she is the first to 

 give up her liberty. Sometimes the male, not content 

 with leaving to her all the trouble of providing for 

 the family, demands from her his daily food, and estab- 

 lishes himself lite a spermatophore on her sexual organs. 

 It is only right to say that in this case, the male sex 

 is far from being the stronger, for he is often less than 

 the tenth or even the hundredth part of the size of the 

 female. At last we see the female lose her claws and her 

 swimming apparatus, while the male keeps his carapace 

 with all his appendages of the senses and of locomotion. 

 The difference between the two sexes is so great in some 

 species, that it would be impossible to imagine that a 

 brother and sister could assume such dissimilar forms, 

 unless we had watched them from the time when they 

 first issued from the egg. The female is a kind of 

 puffed-out worm, and- the male resembles an atrophied 

 acarus. This explains why the female was known so 

 long before the male, whose office is only that of re- 

 production. Nordmann, during his residence at Odessa, 

 was the first to begin these researches, which have been 

 continued by Messrs. Metzger and Claus. 



It is known that the Lerneans attach themselves to 

 their hosts by indissoluble bonds, only becoming para- 



