318 



ZOOLOGY 



SECT. 



far from the middle of the trunk, in an elongated ventral yanylion (Fig. 255, 

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

 eyes (Fig. 257, an.} and to the olfactory organs (?'O. ), and is also connected with 

 two pairs of ganglia in the head, which lie deeply sunk in the mesoderm : all the 

 rest of the nervous system retains its primitive connection with the ectoderm. 



Sensory Organs. On the surface of the body are numerous little papilla? 

 carrying stiff bristle-like processes, and probably serving as organs of touch. 



There are two eyes (Fig. 258), 

 situated one on each side of the 

 dorsal surface of the head : each 

 is globular and contains three 

 biconvex lenses (.), separated by 

 pigment (p. ) and surrounded by 

 rod-like sensory cells (rz. ). Behind 

 the head is a ring-like structure, 

 of the nature of an annular ridge 

 of peculiarly modified and in part 

 ciliated cells (Fig. 257, ro.) : to 

 this an olfactory function has been 

 assigned. 



FIG. 208. Section of eye of Sagitta hexaptera. 

 e/i. epiderm ; L lens ; p. pigment ; rz. visual 

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

 Anatomy, after O. Hertwig.) 



Reproduction. - - The Chseto- 



gnatha are monoecious. The ovaries 

 (Fig. 255, ov., Fig. 256, ovy.} are 

 elongated organs situated one on 

 each side of the trunk-region of 



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

 septum. The testes (Fig. 255, ho. , Fig. 256, t*. ) are similarly situated in the tail- 

 region of the ccelome, 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 (*L ) opening at one end into the ccelome 

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

 or vesicula xeininalis (*&. ), 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 invagination (Fig. 

 259, A). Twoendoderm cells (#.)at the anterior end of the archenteron, i.e. the 



FIG. 259. Three stages i?i the development of Sagitta. bl. blastopore ; at. ccelomic sacs; <L 

 mesenteron ; g. sexual cells ; pm. parietal layer of mesoderrn ; st. stomodamm ; vtn. visceral 

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



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 archenteron and divide, forming a group of four cells (B, <j. ), two of which 

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



