VI 



PHYLUM NEMATHELMINTHES 



319 



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 coelomic 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 : subsequently the cavities of the enteric canal and 

 ccelomic sacs re-appear ; the tail-region of the body-cavity is formed from the 

 posterior, undivided portion of the archenteron. The blastopore (&/.) now closes 

 and an invagination of ectoderm the stomodaeum ($t.) 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 stomodaeum ; 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 ridges. The mus- 

 cular 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. 



APPENDIX TO NEMATHEL- 

 MINTHES. 



1. Family Chcetosomidce. 



This family includes three genera of small 

 worms, Ch<*to*oma (Fig. 260), Tristicocho'ta, and 

 fthabdogcteter, which are sometimes included 

 among the Xematoda. 



The bod}' 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 setae, and there may be a double 

 row of movable chitinoid hooks round the head, 

 reminding us of the " jaws " of Sagitta. The 

 ventral surface bears curious locomot&r rods (f), 

 either hooked or with knobbed ends : by these 

 the animals crawl. The mouth is anterior and 

 terminal, the anus posterior and ventral, and there 

 is a muscular pharynx. The sexes are separate. 

 The male has a single testis : the vas deferens 

 opens along with the anus : there are two penial 

 setae. The female has paired ovaries and a single vagina opening near the 

 middle of the body on the ventral side. 



f- 



Fio. 260. Mature female of 

 Chactosoma claparedii. 

 X 57. a, oesophagus ; b, in- 

 testine ; c, anus ; d, ovary ; 

 e. generative pore ; t, loco- 

 motor rods. (From Shipley, 

 after Metschnikoff.) 



2. Family Echinoderidce. 



Echinochres is a minute marine worm of cylindrical form with a flattened 

 ventral surface. The body is segmented, or divided into rings, eleven or twelve 

 in number, all strongly cuticularised, and most of them bearing spines (Fig. 261). 

 The mouth is placed at the anterior, the anus at the posterior end of the body : 



