VI 



PHYLUM NEMATHELMINTHES 



313 



cells (B, g.), two of which subsequently become the ovaries and two the testes. 

 At the same time two folds of endoderm grow into the archenteron from its 

 anterior end, partly dividing the cavity into three parts a middle division 

 or mesenteron (d) the rudiment of the intestine, and two lateral divisions 

 the metentera, or ccelomic sacs (c. s.) which give rise to the right and left 

 compartments of the coelome of the trunk. From the latter are given off in 

 front a pair of small head-cavities. Owing to the rapid elongation of the 

 embryo in the stages following, all the cavities become for a time obliterated : 



FlG. 264. Tlirr<! stages in the development of Sagitta. bl. blastopore ; r.v. ruMomic sacs ; d. 

 mesenteron ; g. sexual cells ; pm. parietal layer of mesoderm ; st. stomodseum ; vm. 

 visceral layer of mesoderm. (From Lang's Comparatfre Anatomy, after O. Hertwig.) 



subsequently the cavities of the enteric canal and ccelomic sacs reappear ; 

 the tail-region of the body-cavity is formed from the posterior, undivided 

 portion of the archenteron. The blastopore (bl.) now closes and an invagina- 

 tion of ectoderm the stomodseum (st.) takes place at the anterior end, and 

 finally communicates with the mesenteron. 



From this it will be seen that the ectoderm of the gastrula gives rise to the 

 deric epithelium of the adult and to the epithelium of the pharynx, which is 

 therefore a stomodseum ; from the same layer the nervous system arises at a 

 later stage. The epithelium of the intestine arises 

 from the mesial (inwardly-turned) layers of the 

 two endodermal folds. The muscular layer of the 

 body-wall arises from the rest of the endoderm, 

 i.e., that portion of it which remains in immediate 

 contact with the ectoderm. Thus in Sagitta the 

 mesoderm is entirely derived from the endoderm 

 of the gastrula. 



d 



APPENDIX TO NEMATHELMINTHES. 



1. Family Chcetosomatidce. 



This family includes several genera of small 

 marine worms, Chcetosoma (Fig. 265), Tristi- 

 cochcpia, Rhabdoqaster, and others, which are 

 sometimes included among the Nematoda. 



The body is elongated, its anterior region being 

 sometimes dilated to form a head. Either the 

 whole body or the dorsal surface only is beset 

 with fine setse, and there may be a double row of 

 movable chitinoid hooks round the head, remind- 

 ing us of the " jaws " of Sagitta. The ventral 

 surface bears posteriorly curious locomotor rods FIG. 265. Mature female of 

 (/.), either hooked or with knobbed ends: by ^T^opS^!-' 

 these the animals crawl. The mouth is anterior testine ; c, anus ; d, d, 

 and terminal, the anus posterior and ventral, and vaies ' e - generative pore ; 

 there is a muscular pharynx. The sexes are 



