TYPE NEMATHELM1NTHE8. 179 



muscle-cells, and is divided into two lateral chambers by a 

 mesentery (m) running the entire length of the body and con- 

 sisting of two layers surrounding the intestine {d), and inserted 

 into the body-wall dorsally and ventrally, their outer surfaces 

 being lined by a continuation upon them of the peritoneal 

 epithelium. 



No excretory system has been as yet discovered. The 

 nervous system consists of a ganglionic ring surrounding the 

 oesophagus, from which a number of nerves pass forward, 

 while a single nerve-cord (m) passes backwards in the mid- 

 ventral line, dilating at the posterior end of the body into 

 a ganglionic mass. 



The reproductive organs consist in the female of a series 

 of ovaries {ov) attached one behind the other to each mesen- 

 tery above the intestine. In the mesenteries two tubes {ut) 

 pass backwards which receive some of the ova and function 

 as uteri, near the hind end of the body bending ventrally to 

 open into the cloaca, whose wall is invaginated to form a 

 single seminal receptacle. The testes have not yet been 

 found, but two seminal vesicles, corresponding to the uteri of 

 the female, occur and open likewise into the cloaca, which in 

 the male is evertible and serves as a copulatory organ. 



The Affinities of the Nematodes. — The relationships of the Nematodes are 

 exceedingly obscure. Their unsegmented character and the character of 

 the nervous system seem to ally them more closely with the Platyhelminths 

 than with higher forms, but the relationships to any of the known Platy- 

 helminths must be exceedingly remote. The parasitism which occurs so 

 frequently in the group is to be considered as secondary, since so many 

 forms lead a free life and peculiarities of structure can hardly be attributed 

 to degeneration. The Gordiacea stand on a higher plane than the Eu- 

 nematoda, as shown by the possession of a mesentery and the arrange- 

 ment of the reproductive organs and nervous system, which bear some sim- 

 ilarities to those of the Annelids, but their Nematode characteristics are 

 most pronounced. Perhaps the ancestors of the Nematodes are to be found 

 in the yet unknown intermediate forms between the Platyhelminths and 

 Annelids, a view which would account for their similarities in certain 

 respects to both these groups. 



II. Class Acanthocephala. 



This class contains a number of parasitic forms which 

 occur more especially in tho digestive tract of fishes, though 



