298 CaSLHELMJNTIIES. 



In other words the co3lom is .in outgrowth from the archenteron, i.e. is an 

 enteroccele. Second : Tije gonads are derived from a pair of cells in the 

 primitive entoderm, which later are carried into the ccelomic walls. Hence 

 each divides into anterior and posterior cells, the anterior developing 

 into the ovary, the posterior into testes. Hence here the male and female 

 sex cells are beyond doubt descendants of a common mother cell. 



The few species of Chastognathi are arranged in two or three genera, 

 of which SaijUta, represented on our coasts by 6'. elegans* is best known. 

 Upadella. 



Class II. Nemathelminthes. 



Like the flatworms, the roundworms are characterized by their 



shape, they heing thread-like or cylindrical animals whose form is 



the result of the existence of a body cavity in which the viscera are 



so loosely held that on cutting through the muscular body wall 



they will fall out (fig. 259). Since the Nemathelminthes share this 



coelona vi^ith most annelids, the distinction between the two rests 



largely upon negative characters, the roundworms lacking the 



segmentation of the body cavity and tlie corresponding ringing or 



annulation of the body wall. To the Nemathelminthes belong 



three orders, much alike in habits and apipearance but differing 



considerably in structure. Of these the most important are the 



nematodes. 



Order I. Nematoda. 



The nematoda contain numerous species of thread-shaped 

 worms varying from 0.001 to 1.0 metre in length, many of 

 which, through their wide distribution as parasites in plants, 

 animals, and man, jjossess special interest. The outer surface is 

 covered by a tough cuticle secreted by the underlying hypoderniis 

 (fig. 259), a layer corresponding to epithelium and cutis, which in 

 cross-section shows, median and lateral, four thickenings, the 

 dorsal, ventral, and lateral lines. In the lateral lines run the excre- 

 tory vessels, two longitudinal canals which are united near the head 

 by a transverse vessel opening on the ventral surface by an unpaired 

 jiorus excretorius to the exterior. Tliey are related to the co?lom 

 by two giant cells on either side which send processes into the 

 body cavity. Those lateral and median lines divide the muscles 

 (here only longitudinal) into four fields, as \\\ Chretognaths. These 

 muscles are parts of the somatic epithelium, a layer of vesicular 

 cells which by their size (fig. 259) so encroach upon the coalom 

 tliat scarce space is left for the alimentary canal and reproductive 

 organs. A S])lanclinic mesoderm is lacking. 



Tlic alimentary canal begins with a terminal mouth and ciuls 

 A\ith the anus, wliich is ventral aiul in fi'ont of the end of the body. 



