296 ZOOLOGY 



of endoderm grow into the archeiiteron 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 (ca). There 

 is some doubt as to the fate of these lateral divisions. According to the account 

 which is usually accepted they become the right and left compartments of the 

 coelome of the trunk. According to another account, however, their cavities 

 completely disappear and the trunk portion of the coelome arises from a fissure 

 which appears subsequently between the ectoderm and the endoderm ; the 

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

 the archenteron. The blastopore (bl.) now closes and an imagination of ectoderm 

 the stomodffium (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 stomodffium ; 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; their lateral (outwardly-turned) layers 

 form the visceral layer of coelomic epithelium. The muscular layer of the body 

 wall and the parietal layer of ccelomic epithelium arise 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 NEMATHELMINTHES. 



1. Family Chcetosomidce. 



This family includes three genera of small worms, Chcetoxoma, Tristicochceta 

 and Rhabdogaster, which are sometimes included among the Nematoda. 



The body is elongated, its anterior region sometimes dilated to form a head. 

 Either the whole body, or the dorsal surface only, is beset with fine seta?, 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 locomotor 

 rods, either hooked or with knobbed ends : by these the animals crawl. The 

 mouth is anterior- and terminal, the anus posterior and ventral ; 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 seta?. The female 

 has paired ovaries and a single vagina opening near the middle of the body 

 on the ventral side. 



2. Family Echinoderidce. 



Echinoderes 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. The 

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

 former opens into a sac, which can be everted so as to form a proboscis or 

 introvert, and is armed with spines. The enteric canal consists of a muscular 

 pharynx and a straight intestine. A pair of sacs opening by ciliated ducts on the 

 tenth segment appear to be excretory organs. The sexes are separate : the gonads 

 are paired sacs opening at the posterior end of the body. 



3. Family Desmofscolecidce. 



1)< xinoscolex is also a minute marine worm, having a globular head and a 

 variable number of segments. The head bears four movable chitinoid rods or 

 setae, and a pair of similar structures occurs on many of the other segments. The 

 terminal mouth leads by a muscular pharynx into a straight intestine : the anus 

 is dorsal in position. The animal is dioecious: the gonads have the form of 

 simple sacs, the testis opening along with the anus, the ovary on the ventral 

 surface anterior to the anus. The male has a pair of penial seta'. 



