/. CH^TOGNATBI. 



297 



pair of fused cerebral ganglia (fig. 262), in the trunk segment a 

 large ventral ganglion, and these are connected 



" m by long cesophageal commissures. Of interest, 

 because characteristic of nematodes and many 



f annelids, are' the relations of the musculature, 

 which consists of longitudinal fibres alone. 

 The body cavity is lined with epithelium (fig. 

 260), which, so far as it abuts against the ali- 

 mentary tract, is called splanchnic (or visceral) 

 mesoderm; that on the side of the ccelom to- 

 wards the ectoderm is the somatic mesoderm. 

 " The muscles arise from the latter layer and are 

 divided into four fields, right and left dorsal, 

 right and left ventral. The sex cells also arise 

 from the epithelium of the coalom, the eggs 

 r in the trunk segment, the sperm in the tail. 

 The eggs are carried to the exterior by special 

 ducts. The sperm-forming cells early lose 

 their connexion with the epithelium, fall into 

 , the ccelom, where they develop the spermato- 

 zoa. These are carried out by canals which 

 by their relations to the co3lom recall the ne- 

 phridia of the annelids. 



171 



FIG. 261. FIG. 262. 



FIG. 261. Sagitta hexaptera, ventral view. (After O. Hertwig.) a, anus; &<?, ventral 

 ganglion; d, intestine; //, fin; /to, testes; m, mouth; ov, ovary; oud, oviduct: s?>, 

 seminal vesicle; sc, cecophageal commissure; sfl, tail fin; s/, sperm; wo, female 



FIG. 262. Head of Sagittn bipunctata, dorsal view. (After O. Hertwig.) an, nerve 

 to ati, eye; g, brain; gh, bristles; ?-?i, nerves to ro, olfactory organs; sc, oesophageal 

 commissure. 



The development of Sagitta is significant from two points of view. 

 The archenteron (fig. 108) is divided by lateral folds into an unpaired 

 middle portion and two paired lateral chambers ; the first is the defini- 

 tive digestive tract, the latter the anlagen of the ccelomic diverticula. 



