312 



ZOOLOGY 



SECT. 



Reproduction. -The Chsetognatha are monoecious. The ovaries (Fig. 

 260, ov., Fig. 261, 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. 260, ho., Fig. 261, ts.) are similarly 

 situated in the tail-region of the ccelome, and have the form of narrow ridges 



d.ef}tkrrt 



Fio. 261. Sagitta bipunctata. Transverse sections, A, of trunk ; B, of tail. coel. coelome ; 

 coel. epthm. layer of nuclei of the muscle-cells formerly regarded as a coelomic epithelium ; 

 rf. epthm. deric epithelium, diagrammatically shown as a single layer ; /. tin ; int. intestine ; 

 m. muscles ; ovy. ovary ; ts. testis. (After Hertwig.) 



from which immature seminal cells are given off and develop into sperms in 

 the ccelome. The spermiducts or vasa deferentia are delicate tubes (si.) 

 opening at one end into the coelome by a ciliated funnel-like extremity, and 

 at the other end dilating into a reservoir or vesicula seminalis (sb.), which 

 opens externally in the posterior region of the tail. 



FIG. 262 V Head of Sagitta bipunctata, 



from above, an. optic nerve ; au. eye ; g. 

 brain ; gh. hooks ; rn. olfactory nerve ; ro. 

 olfactory organ ; sc. cesophageal connective. 

 (From Lang's Comparative Anatomy, after 

 Hertwig.) 



FIG. 263. Section of eye of Sagitta hexap- 

 tera. ep. epiderm ; I. lens ; p. pigment : 

 rz. visual cells ; st. rods. (From Lang's 

 Comparative Anatomy, after O. Hertwig.) 



Development. Internal impregnation takes place, and the oospenn, 

 segmenting completely and regularly, forms a typical gastrula by invagination 

 (Fig. 264, A). Two endoderm cells (g.) at the anterior end of the archenteron, 

 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 considerable importance, as will be seen hereafter. Before long 

 these cells migrate into the archenteron and divide, forming a group of four 



