PARASITIC ROUND WORMS 515 



uous feature in most nematodes. It lies not far from the excretory 

 pore, a short distance behind the anterior end of the esophagus. 



A cross section of the body shows on the exterior the thick 

 non-cellular cuticula; within it the hypoderm or sub-cuticula 

 which is cellular but without cell walls. This layer is thin except 

 at the median and lateral fields which are visible externally as 

 faint streaks and hence often called "lines"; here it projects inward 

 between the muscle cells. The major part of the body wall con- 

 sists of the muscular layer, a single layer of large cells with longi- 

 tudinal but no circular or cross fibers; these cells have a con- 

 spicuous protoplasmic body on the inner side next the body cavity. 

 The muscle layer is divided into four areas separated by the median 

 and lateral fields of the hypodermis; rarely the presence of sub- 

 median fields makes eight such muscle areas. Each of the four 

 muscle areas may contain many muscle cells (the Polymyaria) 

 or be limited to a longitudinal series of two muscle cells (the 

 Meromyaria) . 



The cuticula of nematodes is usually said to be " chitinous " but 

 as this layer is soluble in alkalis, digested by the action of en- 

 zymes, and contains a very high percentage of nitrogen, it is not 

 chitin; it has been correctly designated as cornein by Reichard. 

 Glycogen occurs in large amount in nematode tissues and is sup- 

 posed to furnish them oxygen and energy. 



The body cavity is large but not lined by a peritoneal epithe- 

 lium. It is in fact formed by the breaking down of connective 

 tissue cells, the remnants of which may still be observed in well 

 preserved specimens, especially at the anterior end. Both repro- 

 ductive and digestive organs are free in this cavity since mesen- 

 teries are lacking. In full-grown worms the space of the body 

 cavity is almost entirely filled by the greatly enlarged and much 

 convoluted reproductive organs which press upon each other, the 

 alimentary canal, and the body wall so as to leave only small 

 irregular cavities here and there. 



The reproductive system is exceedingly simple. In both sexes 

 it has the form of a long tube in which the various regions are 

 continuous and only slightly distinguished from each other in form. 

 The fine inner end of the tube produces the reproductive cells, 



