298 CCELHELMINTHES. 



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

 enterocoele. Second : The gonads are derived from a pair of cells in the 

 primitive entoderm, which later are carried into the coelomic 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 Chaetognathi are arranged in two or three genera, 

 of which JSagitta, represented on our coasts by 8. elegans* is best known. 

 Spadella. 



Class II. Nemathelminthes. 



Like the flat worms, the roundworms are characterized by their 

 shape, they being 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 

 coelom with most annelids, the distinction between the two rests 

 largely upon negative characters, the roundworms lacking the 

 segmentation of the body cavity and the corresponding ringing or 

 annulation of the body wall. To the Nemathelminthes belong 

 three orders, much alike in habits and appearance 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, possess special interest. The outer surface is 

 covered by a tough cuticle secreted by the underlying hypodermis 

 (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 

 porus excretorius to the exterior. They are related to the coelom 

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

 body cavity. These lateral and median lines divide the muscles 

 (here only longitudinal) into four fields, as in Chsetognaths. These 

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

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

 that scarce space is left for the alimentary canal and reproductive 

 organs. A splanchnic mesoderm is lacking. 



The alimentary canal begins with a terminal mouth and ends 

 with the anus, which is ventral and in front of the end of the body. 



