II. NELIATHELMINTHES: NEMATODA 263 



called splanchnic mesoderm; that on the side of the coelom towards the ecto- 

 derm is the somatic mesoderm. 1 he muscles arise from the latter and are 

 divided into four fields, right and left dorsal, right and left ventral. The sex 

 cells also arise from the epithehum of the calom, the eggs in the trunk segment 

 (fig. 241), the sperm in the tail. The eggs are carried to the exterior by special 

 ducts. The sperm-forming cells early fall into the ccelom, where they develop 

 the spermatozoa. These are carried out by canals which recall the nephridia 

 of the annelids. 



The development of Sagitta is significant from two points of view. The 

 archenteron (fig. 109) is divided by lateral folds into an unpaired middle portion 

 and two paired lateral chambers; the first is the definitive digestive tract, the 

 latter the anlagen of the ca'lomic diverticula. In other words, the ccelom is an 

 outgrowth from the archenteron, i.e., is an enteroccele. Second: The gonads are 

 derived from a pair of cells in the primitive entoderm, which later are carried 

 into the coelomic walls. Here 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 are arranged in two or three genera, of which Sagitta, 

 represented on our coasts by 5. elegans,*- is best known. Spadella. 



Class II. Nemathehninthes. 



Like the flatworms, 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. 244). 

 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 corre- 

 sponding ringing or annulation of the body wall. The body cavity 

 apparently is different since the splanchnic wall is lacking, the space 

 lying between mesoderm and entoderm (pseiidoccvle). To the Nemathel- 

 minthes 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 o.ooi to i.o 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 

 siibcutiatla, a fibrous ectodermal syncitium (fig. 240), which in cross- 

 section sho^vs four thickenings, the dorsal, ventral, and lateral lines. In 

 the lateral lines run the excretory vessels, two longitudinal canals, united 

 near the head by a transverse vessel opening on the ventral surface by aj» 



