VI 



PHYLUM NEMATHELMINTHES 



287 



in being hermaphrodite. The eggs are laid and the embryos pa.ss 

 from the lungs into the enteric canal of the host, are expelled with its 

 faeces, and develop in water into a sexual Nematode, called the 

 Rhabditis-form, in which the sexes are separate: in this the ferti- 

 lised eggs develop in the body of die female, and, when fully fonnr<l. 

 make their way through the wall of the uterus and proceed to devour 

 the whole of the maternal tissues, leaving nothing but the cuticle. 



Fir;. '233. Development of Ascaris nigrovenosa. Wj>. blastopore ; ect. ectoderm : < ,nl. 



derm ; ent. enteron ; !.:.*. mesoderm ; ii. nervous sj-stem ; xtdm. stomodteum. (From Korschelt 

 and Heider, after Goette.) 



Being set free, they live in mud until they succeed in gaining 

 access to a frog's mouth, when they pass into the lung, develop 

 hermaphrodite reproductive organs, and so re-commence the cycle. 

 It will be seen that we have here a peculiar form of alternation 

 of generations, distinguished not by the alternation of a sexual 

 with an asexual form (metagenesis) as in Hydrozoa, but by the 

 alternation of a hermaphrodite with a dioecious form. This type 

 of alternation of generations is distinguished as heterogeny. 



