CHAETOGNATHA. 



589 



It is further divided by two transverse septa into three successive 

 chambers : one in the head, another in the trunk, and the third 

 in the tail (Fig. 468). These chambers are indications of segmenta- 

 tion, though the exact mode of their origin from the original 

 continuous coelom has not been made out. Of these three coelomic 

 chambers or segments, the posterior alone has, so far as is known, 

 a communication with the exterior through the male generative 

 ducts. 



Generative organs. The animals are hermaphrodite. The female 

 organs are contained in the middle segment of the body, and the 

 oviduct opens on each side of the anus at the junction of the middle 

 and caudal regions. The ovaries are placed one on each side 

 (Fig. 468) ; they are solid and are attached to the wall of the 

 body, projecting into the body-cavity. The oviducts are narrow 



FIG. 471. Three embryos of Sagitta (from Lang, after Hertwig). U blastopore ; ud arch- 

 enteron ; g primitive generative cells ; vm splanchnic, pm somatic (parietal) layer of 

 mesoderm ; d rudiment of intestine ; cs coelomic sacs ; st stomodaeum (ectodermal ingrowth 

 which forms the anterior part of the alimentary canal). 



tubes extending along the outer sides of the ovaries and lined by 

 an epithelium. They end blindly in front, and open to the exterior 

 behind; they sometimes contain spermatozoa. The ova are not 

 dehisced into the coelom, and it is not known how they get into 

 the oviducts, which appear to be much too narrow to contain 

 them. 



The testes are thickenings of the parietal coelomic epithelium in 

 the anterior part of the caudal chambers of the body-cavity. Cells 

 break off from them and fall into the body-cavity, where they develop 

 into spermatozoa. The male generative ducts (Fig. 468) are paired 

 tubes opening on the side of the body in the caudal region, and 

 internally by a ciliated opening into the caudal region of the coelom. 

 The vesicula seminalis is a dilatation on the course of the short 

 vas deferens. 



