PHYSIOLOGY OF THE INVERTEBRATA. 39 



(i) ^X\\Q Xcmatoidca. — The "thread-worms" possess elon- 

 gated, rounded bodies. They are not segmented organisms. 

 The anterior end is sometimes furnished either with liooks 

 and spines within the oral cavity, or with papillae around 

 the mouth. The mouth leads into a muscular 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 Ncmatorliyncha. — This order contains the follow- 

 ing genera among others: Clicctonotus, Chcvtura, Dasyditis, 

 and TurhancUa. These organisms are allied to the Rotifera, 

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

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

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

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

 •of the Gastrotricha,^ the closest affinities of the Nemato- 



rhyncha are with the Ncmatoidca, and I therefore place them 

 among the jVematoscolices." 



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

 particularly Uchinorhyjichus, are parasitic, for in the sexless 

 condition they infest the Invcrtchrata, while in the sexual 

 state they are found infesting the Vcrtebrata. 



There is neither a mouth nor an alimentary canal in 

 Echinorliynchus. 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 Ch.eto(JNatha. 



This class of the Arthrozoic Series is represented by only 

 one genus — the Scujitta. 



* Mtrmis has no anus. 



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

 ilivided. 



