CHAPTER VII 



PHYLUM NEMATHELMINTHES 



The Nemathelminthes or roundworms are threadlike or eyhn- 

 drical worms which have a body cavity and lack segmentation. 

 The body is covered with a heavy cuticula. Highly developed 

 sensory and locomotor organs are wholly lacking. Reproduction 

 is invariably sexual, and in the course of direct development 

 distinctive free larval stages are lacking. Since the splanchnic 

 layer of the mesoderm is wanting, the body cavity is not a true 

 coelom but is termed a pseudocoel. Three classes are usually 

 recognized as belonging to this phylum, the Nematoda (or thread- 

 worms), the Gordiacea (or hair snakes), and the Acanthocephala 

 (or spiny headed worms). There is considerable doubt as to the 

 correct location of the Acanthocephala in the system, for evi- 

 dences indicating flatworm relationships are not wanting. The 

 three classes differ so fundamentally in structure that it seems 

 best to offer individual treatment of the groups rather than 

 discuss characteristics of the phylum further. 



Class Nematoda 



The nematodes or nemas, as they are frequently called, have 

 characteristically elongated, cylindrical bodies (Fig. 70 B) 

 covered by a resistant cuticula. They occupy almost every con- 

 ceivable habitat capable of supporting life. Fresh-water, marine, 

 and soil-inhabiting species are extremely numerous and as para- 

 sites both of plants and of animals they are of high importance. 

 In length, they range from a fraction of a millimeter to more than 

 a meter. There are no appendages and no segmentation, though 

 in some free-living forms striations, cuticular scales, or bristles 

 may give a superficial appearance of segmentation. 



Histology. — In section, the body wall is seen to be composed of 

 an external non-cellular layer, the cuticula, directly beneath 

 which lies the subcuticula or hypoderm. A layer of partially 

 differentiated muscular cells lines the outer wall of the body cav- 

 ity and comprises the chief bulk of the body wall. Each of these 



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