CCELHELMINTHES 



217 



representatives of the class and, with the exception of the thorn-headed 

 worms, all of the species of medical interest. 



Order I. Nematoda. — Alimentary canal present. 



Order II. Acanthocephala. — Thorn-headed. Alimentary canal ab- 

 sent. 



Order I. Nematoda 



Xemathelminthcs (p. 216). — The order Nematoda includes numerous 

 species having a wide distribution as parasites of animals and plants. 

 The outer surface of the 

 body is covered by a 

 tough chitinous cuticle 

 which is secreted b}' an 

 underlying layer corre- 

 sponding to the epithe- 

 lium and derma. The 

 cuticular surface may be 

 plain, striated, or more or 

 less mottled. Transverse 

 section of the body wall 

 shows four thickenings — 

 two median and two 

 lateral — corresponding to 

 the dorsal, ventral, and 

 lateral lines which are 

 cUsposed longitudinally. 

 Within the lateral thick- 

 enings are contained the 

 two excretory vessels 

 which, in the vicinity of 

 the head, unite by a 

 transverse commissure 

 reaching the exterior on 

 the ventral surface. The 

 muscles are a layer of 

 vesicular cells derived 



from the epithelium of the outer coelomic wall. They are divided by 

 the lateral and median lines into four fields, and so project into the 

 coelom as to occupy considerable of its space (Fig. 119). 



The digestive system is simple and complete, beginning with the 

 anterior terminal mouth and ending in an anus which is ventral and 

 close to the caudal extremity of the body, A muscular esophagus suc- 

 ceeds the mouth, soon expanding to form a bulbous sucking organ 

 lined throughout with a cuticular layer. From this point to 



Fig. 119. — Transection of t)ody of Ascaris equi (fe- 

 male), showing cuticular wall, muscle cells and proto- 

 plasmic processes extending into ccclom, transversely 

 cut portions of ovary and uterus, and intestinal canal in 

 center (from microphotograph by Hoedt). 



the 



