102 ANIMAL PARASITES AND MESSMATES. 



of form from the cradle to the tomb, by following them 

 from fish to fish, or rather from stomach to stomach. 

 In fact these parasites are perpetually on their journey, 

 and constantly changing their host, and at the same 

 time their dress and mode of locomotion, so that 

 frequently, at the end of their voyage, they preserve 

 only shapeless rags to cover their eggs or their offspring. 



That which adds still more to the difficulty of recog- 

 nizing them is, that while young they are often enveloped 

 in swaddling clothes which nevertheless permit them to 

 wander freely; then in a simple robe, in keeping with 

 the home which shelters them ; and at last in a wedding 

 dress, which hides the . eggs and the apparatus which 

 produces them. The nymph in her virgin condition has 

 none of the attributes of future maternity. 



It is in this category that we find the Distomes, so 

 common in all the classes of the animal kingdom. This 

 is not all : frequently, among these various forms, these 

 animals when young produce little ones, which in no 

 respects resemble the others, and are not even formed 

 in the same manner. As soon as they quit their swadd- 

 ling-clothes, they increase by gemmation, and without 

 sexual union, while those which are produced from buds 

 increase sexually. Thus the daughterdoes not resemble 

 her mother, but her grandmother. This phenomenon 

 has been known by the name of alternate generation; 

 we have called it digenesis. 



But all parasites do not resemble those distomes, 

 which change several times both their host and their 

 costume. We find some of them, which the mother 

 deposits with care in the body of a neighbour, and which 

 pass all their early life in the viscera of an alien mother. 



