PHYSIOLOGY OF THE INVERTEBRATA. 39 



(i) The Nematoidea, — The "thread-worms" possess elon- 

 gated, rounded bodies. They are not segmented organisms. 

 The anterior end is sometimes furnished either with hooks 

 and spines within the oral cavity, or with papillae around 

 the mouth. The month leads into a niuscular pharynx, lined 

 with chitin, which then proceeds into a narrow oesophagus — 

 the latter passing into a long intestine which terminates in 

 an anus * situated ventrally. There is no dilatation of the 

 alimentary canal to form a stomach. 



(2) The Nematorhyncha. — This order contains the follow- 

 ing genera among others : Chcetonotus, Chcetwa, Dasyditis, 

 and Turbanella. These organisms are allied to the Botifera, 

 " but they differ from them in the absence of a mastax, and 

 in the disposition of the cilia, which are restricted to the 

 ventfal surface of the body." Professor Huxley says : " On 

 the whole, however, I think that, notwithstanding the cilia 

 .of the Gastrotricha,^ the closest afBnities of the Nemato- 

 rhyncha are with the Nematoidea, and I therefore place them 

 among the NeTnatoscolices." 



(3) The Acanfhocejphala. — The animals of this order, and 

 particularly Echinorhynchus, are parasitic, for in the sexless 

 condition they infest the Invertebrata, while in the sexual 

 state they are found infesting the Vertebrata. 



There is neither a mouth nor an alimentary canal in 

 JHchinorhynchus. This is another example of reversion to a 

 low type of digestion. No doubt nutrition is performed by 

 the absorption or imbibition of fluid nutriment through the 

 external walls of the body. 



The Oh^tognatha. 



This class of the Arthrozoic Series is represented by only 

 one genus — the Sagitta. 



* Mermis has no anus. 



t One of the two groups into which the Nematorhyncha have been 

 divided. 



