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 glandular nature, 

 though their functions are still veiled in obscurity. Here also occur 

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

 of an involuntary nervous system. 



Cylindroid Conoid 



Fusiform Clavate Dorylaimoid Oxyuroid Rhabditoid Tylenchoid Aphelenchoid 

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



The intestine is a tubular canal extending from the esophagus 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 histology of this 

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

 markedly different in structure and chemical reaction from those 

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

 tion will show that some of the anterior cells of the intestinal tube 

 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 Dorylaimus and its 

 congeners, just preceding the rectum there is a short very distinct 

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

 the definiteness of its structure its function is unknown. Emptying 

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

 lular glands, called anal glands, perhaps serving as accessories in defe- 



