164 



Nemathelminthes. 



Fiff. 126. 



Fig. 127. 



possess an excretory apparatus, similar to that of the 

 Platyhelminths, i.e., provided with the typical terminal branches. It 

 opens into the oviduct or vas deferens*. Ova are found free in the 

 body-cavity of the female in different stages of development; only 

 one oviduct is present, which, although somewhat complicated in 

 structure, is, essentially, a canal open at both ends ; the ova enter by 

 the anterior opening : the posterior one is effei'ent, and opens at the 

 hind end of the body. The male is usually smaller than the female, 

 and possesses two testes; their efferent ducts unite to form a common 

 vas deferens, beset with glands, and opening at the posterior end of 

 the body in a tolerably wide eversible bursa. 



All the Acanthocephala belong to the one genus Echinorhynchus ; 

 thi'Y live, in the adult state, in the alimentary canal of Vertebrata, 



with the proboscis 

 fixed in the mucous 

 membrane, and they 

 feed upon the contents 

 of the intestine. Their 

 development is 

 of interest. The eggs 

 of E. Proteus (one of 

 the forms whose life- 

 history is best known) 

 living in the gut of 

 different Fresh-water 

 rish, escape with the 

 excreta of the Fish, 

 and are consumed by 

 a small Crustacean, 

 Gammarus pulex, in 

 whose alimentary 

 canal the elongate 

 larvae hatcht out. 

 The front end of the 

 larva is provided with a boring apparatus consisting of ten spines 

 {see Fig. 127), by means of which it traverses the intestinal wall 

 into the body-cavity of the Crustacean. Here it wanders about, 

 grows, and graduallj'' assumes the form of the adult. If the 

 Crustacean be eaten by a Fish, the parasite gets into the alimentary 

 canal, and here attains sexual maturity. E. gigas, the female of which 

 may attain a length of 50 c/m. (the male only 9 c/m.), lives, in the 

 adult state, in tlie digestive tract of the Pig; as a larva, in the larva' 

 of the Rose-chafer {Cetonia anrata) and other Lamellicornia. 



* The excretory organs do not, as in the Flatworms, spread over the whole body, 

 but are limited to a small region. 



t It is noteworthy that the oviduoal canal has a lateral opening, through which 

 the irnripe eggs, taken up by the oviduct, pass again into the body-cavity, whilst 

 the ripe ones pass through the canal. 



Fig. 126. 

 Fig. 127. 



Echinorhyrchus. — After Leuokart. 

 Larva of one of the Acanthocephala. — Alter 

 Kaiser. 



