262 



CCELHELMINTHES 



(fig. 241, 0) are specialized parts of the coelomic epithelium and their prod- 

 ucts 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 absent. Little in general can 

 be said of the nervous system; details will be given 

 in connection with the separate classes. 



sc 



f 



\A 



ovd 



-fl 



-ov 



Class I. Chaetognathi. 



These marine forms, a half to two inches long, per- 

 fectly transparent, live at the surface of the sea, preying 

 on other animals, and from their shapes and rapid motions 

 deserve the name Sagitta arrow given some forms. 

 The animals swim by means of horizontal fins, one sur- 

 rounding the tail and one or two pairs on the sides of the 

 trunk (fig. 42). On either side of the mouth are strong 

 bristles used in seizing prey (Chaetognath, bristle-jaw). 

 Internally the body is separated into head, trunk, and 

 tail, by transverse septa which divide the ccelom into 

 corresponding chambers. Each segment of the coelom 

 again is divided into right and left halves by a mesen- 

 tery (fig. 241), supporting the straight intestine, running 

 lengthwise through it and terminating at the anus at the 

 end of the trunk segment. 



The nervous system is entirely ectodermal. In the 

 head is a pair of fused cerebral ganglia (fig. 243), in the 

 trunk segment a large ventral ganglion, and these are 

 connected by long oesophageal commissures. Of inter- 

 est, because characteristic of nematodes and many an- 

 nelids, are the relations of the musculature, which 



FIG. 242. FIG. 243. 



FlG. 242. Sagitta hexaptera, ventral view (after O. Hertwig). a, anus; bg, 

 ventral ganglion; d, intestine;//, fin; ho, testes; ;, mouth; oi 1 , ovary; m>d, oviduct; sb, 

 seminal vesicle; sc, cesophageal commissure; sfl, tail fin; si, sperm; wo, female opening. 



FIG. 243. Head of Sagitta bipunctaia, dorsal view (after O. Hertwig). an, 

 nerve to au, eye; g, brain; gh, bristles; rn, nerves to ro, olfactory organs; sc, cesophageal 

 commissure. 



consists of longitudinal fibres alone. The body cavity is lined with epi- 

 thelium (fig. 241), which, where it abuts against the alimentary tract, is 



