146 



ROUNDWORMS. PHYLUM NEMATODA 



iinimi 



CUV. . 



mi- 



nt. 



Sho. 



yrtgt, 



{=Trichnyis) and Enterohius (=Oxyuris) (Fig. 108) are medically 

 i]H)rtant inhabitants of the human gut, the former about an 



inch long, the latter half an inch in the 

 female and an eighth in the male. Eggs 

 pass out with the host's faeces, and may be 

 swallowed with raw vegetables. Those of 

 Enterohius are ripe as soon as they reach 

 the exterior, so that reinfection by scratch- 

 ing often takes place. 



9. A free bisexual generation alternates 

 with a parasitic hermaphrodite. — The her- 

 maphrodite stage of Rhabdias bufonis 

 {=Rhabdonema nigrovenosum) lives in the 

 lungs of the frog. It is protandrous. 

 Embryos escape by the glottis and cloaca, 

 and become sexual adults. The young 

 produced by these wander, and may be 

 swallowed by another frog. 



The nematodes are an isolated and uni- 

 form group. They show certain superficial 

 resemblances to arthropods (ecdysis, absence 

 of cilia, absence of coelom, unflagellated 



Fig. los.-Oxyuns, some- sperms), but these are not enough to justify 

 what diagrammatic, to the Suggestion that the parasitic forms 



show arrangement of , . . , , t ji r 



are degenerate arthropods and the free- 

 living species descended from the parasites. 

 There are many important differences, and 

 it seems best to regard the nematodes as 

 fairly primitive forms which never pos- 

 sessed a coelom. Their success at their 

 own level is obvious. 



afu 



organs. 



A, Male ; B, Female. 



an.. Anus ; g.o., genital opening ; 

 int., intestine ; oes.b., bulb 

 of oesophagus ; ov., ovary ; 

 ph., pharynx ; /., testis ; ut., 

 uterus; v.d., vas deferens; 

 vag. vagina. 



