PHYLUM NEMATHELMINTHES 



295 



VI 



the dorsal side of the pharynx is a comparatively large brain (Fig. 241, y), which 

 sends off on each side a long nerve-cord, the cesophageal connective (sc. ). The two 

 connectives sweep round the enteric canal and unite on the ventral surface, not 

 far from the middle of the trunk, in an elongated ventral ganglion (Fig. 239, 

 by.), from which numerous nerves are given off. The brain sends nerves to the 

 eyes (Fig. 241, an.) and to the olfactory organs (ro.), and is also connected with 

 two pairs of ganglia in the head, 

 which lie deeply sunk in the meso- 

 derm : all the rest of the nervous 

 system retains its primitive con- 

 nection with the ectoderm. 



Sensory Organs. On the 

 surface of the body are numerous 

 little papilla? carrying stiff bristle- 

 like processes, and probably serv- 

 ing as organs of touch. There 

 are two eyes (Fig. 242), situated 

 one on each side of the dorsal sur- 



face of the head : each is globular 



and contains three biconvex lenses 



(I. ), separated by pigment (p. ) and 



surrounded by rod-like sensory 



cells (rz.). The dorsal surface of 



the head also bears an annular 



ridge of peculiarly modified and in part ciliated cells (Fig. 241, ro.) : to this an 



-olfactory function has been assigned. 



Reproduction. The Chffitognatha are monoecious. The ovaries (Fig. 239, or., 

 Fig. 240, ovy. ) are elongated organs situated one on each side of the trunk-region 

 of the ccelome, and opening by a narrow oviduct just in front of the posterior 

 septum. The testes (Fig. 239, ho., Fig. 240, ts.) are similarly situated in the tail- 

 region of the coelome, and have the form of narrow ridges from which immature 

 seminal cells are given off and develop into sperms in the coelome. The spermi- 

 ducts or vasa deferentia are delicate tubes (si. ) opening at one end into the ccelome 

 by a ciliated funnel-like extremity, and at the other end dilating into a reservoir 

 or vesicula seminal is (sb. ), which opens externally in the posterior region of the tail. 



Development. Internal impregnation takes place, and the oosperm, seg- 

 menting completely and regularly, forms a typical gastrula by imagination (Fig. 



FIG. 242. Section of eye of Sagitta hexaptera. 

 ep. epiderm ; 1. lens ; p. pigment ; rz. visual 

 cells; st. rods. (From Lang's Comparative 

 Anatomy, after Hertwig.) 



FIG. 243. Three stages in the development of Sagitta. bl. blastopore ; cs. coelomic sacs ; d. 

 mesenteron ; g, sexual cells ; p,n. parietal layer of mesoderm ; st. stomodseum ; vrn. visceral 

 layer of mesoderm. (From Lang's Comparative Anatomy.) 



243, A). Two endoderm cells (g.) at the anterior end of the archeiiteron, i.e. the 

 end opposite to the blastopore, soon increase greatly in size, and are the rudiments 

 of the gonads. This precocious differentiation of the sex-cells is a point of con- 

 siderable importance, as will be seen hereafter. Before long these cells migrate 

 into the archeiiteron and divide, forming a group of four cells (B, y. ), two of which 

 subsequently become the ovaries and two the testes. At the same time two folds 



