/. CH^fJTO GXA TUI. 



29T 



pair 01 



fused 



ovd.- 



cerebral ganglia (fig. 2fiv!). in the trunk segment a 

 large ventral ganglion, and these are connected 



"'"• by long (esophageal commissures. Of interest, 

 because characteristic of nematodes and many 

 annelids, are the relations of the musculature, 

 which consists of longitudinal fibres alone. 

 The body cavity is lined with epithelivim (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 la3'er and are 

 divided into four fields, right and left dorsal, 

 right and left ventral. The sex cells also arise 

 from the epithelium of the coelom, the eggs 

 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 epitheliiim, fall into 

 the coelom, where they develoj) the spermato- 

 zoa. These are carried out by canals which 

 by their relations to the ccelom recall the ne- 

 phridia of the annelids. 



-b3 



-n 



-ov 



CTV 



-<rl 



FiQ. 261. I'lfi. 2ii~'. 



Fio. 261.— Srtc/iMo fteo-apfem, ventral view. (After O. Hertwig.) rr, anus; />r/, ventral 



ganglion: d. intestine; //, fin; ho, testes; vt, mouth; ov, ovary; otW, oviduct; s)i, 



seminal vesicle; Si\ cesophageal commissure; sjl, tail tin; si, sperm; mo, female 



opening. 

 Fig. 262,— Head of Sagitta hlpunclala, dorsal view. (After O. Hertwig.) an, nerve 



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



commissure. 



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

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

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

 tive digestive tract, the latter the aidagen of the ewlomio diverticula. 



