FREE-LIVING NEMATODES 



469 



mentary canal a distinct constriction, known as the cardiac con- 

 striction. In the immediate vicinity of this constriction small 

 organs are sometimes found, apparently of a ^dimdular naturr, 

 though their functions are still veiled in obscurity. Here also (Kcur 

 definite nerve cells which are probably to be regarded as the center 

 of an involuntary nervous system. 



Cylindroid Conoid 



() 



& 



Fusiform Clavate Dorylaimoid Oxyuroid Rhabditoid lylcnthui.! Apbclcndnid 

 Fig. 770. Forms of the esophagus. (After Cobb.) 



The intestine is a tubular canal extending from the csophap^J^ to 

 near the anus. Usually rather uniform in diameter, it is occasion- 

 ally somewhat expanded just behind the esophagus to form a 

 rudimentary stomach, if one may judge from the histoloi^y (^f this 

 part of the organ. The cells at this part of the intestine are often 

 markedly different in structure and chemical reaction from thosc^ 

 farther back. In almost any species a sufficiently careful examina- 

 tion will show that some of the anterior cells of the intestinal tul)c 

 differ from those farther back, and hence it appears certain that the 

 anterior part of the intestine serves a digestive function, while the 

 remaining part serves as an intestine proper. There are also well 

 differentiated cells in the wall of the posterior part of the intestine, 

 indicating here also a subdivision of functions. 



The intestine ends in a short tubular conoid region leading to 

 the anus, and known as the rectum. This part is more or less 

 muscular and serves to extrude the feces. In Dor vidimus and its 

 congeners, just preceding the rectum there is a short \ery distinct 

 part of the alimentary canal known as the prc-rectum. In spite oi 

 the definiteness of its structure its function is unknown. Kmpt\in- 

 near the anus there are usually to be found a number of small unicel- 

 lular glands, called anal glands, perhaps ser\'ing as accessories in defc- 



