296 



CCELHELMINTHES. 



specialized parts of the ccelomic epithelium and their products are 

 usually carried to the exterior by the nephridia (more rarely by 

 special ducts), so that here, as in vertebrates, we can speak of a 

 urogenital system. A closed blood system is now present, now 



FIG. 260. Transverse section of Sagitta bipunctata and a bit of the body wall more 

 enlarged. (After O. Hertwig.) c, coelonx; dd, entoderm; d/, splanchnic meso- 

 thelium; e, epidermis; w, somatic mesoderm (muscles and epithelium); o, ovary. 



absent. Little in general can be said of the nervous system: 

 details will be given in connexion with the separate classes. 



Class I. Chaetognathi. 



These marine forms, a half to two inches long, perfectly trans- 

 parent, are well adapted to serve as an introduction to the ccelomate 

 worms. They live at the surface of the sea, preying on other ani- 

 mals, and from their shapes and rapid motions deserve the name 

 Sagitta arrow given some forms. The animals swim by means 

 of horizontal fins, one surrounding the tail and one or two pairs 

 on the sides of the trunk (fig. 261). On either side of the mouth 

 is a lobe bearing strong bristles used in seizing prey (Chaeto- 

 gnathi, bristle-jaw). Internally the body is divided into three 

 segments, head, trunk, and tail, by transverse septa which divide 

 the coelom into corresponding parts. Each segment of the cce- 

 lorn again is divided into right and left halves by a mesentery 

 (fig. 260), supporting the straight intestine, running lengthwise 

 through it. The intestine terminates at the anus at the end of the 

 trunk segment. 



The nervous system is entirely ectodermal. In the head is a 



