624 CIIORDATA. 



reader is referred to pp. 355 360 and to the literature there 

 referred to. 



In the Chaetopoda and the Tracheata the body cavity arises 

 as a series of paired compartments in the somites of mesoblast 

 (fig. 350) which have at first a very restricted extension on the 

 ventral side of the body, but eventually extend dorsalwards and 

 vcntralwards till each cavity is a half circle investing the 

 alimentary tract ; on the dorsal side the walls separating the two 



FIG. 350. LONGITUDINAL SECTION THROUGH AN EMBRYO OF AGELINA 

 LABYRINTHICA. 



The section is taken slightly to one side of the middle line so as to shew the rela- 

 tion of the mesoblastic somites to the limbs. In the interior are seen the yolk 

 segments and their nuclei. 



i 16. the segments ; pr.l. procephalic lobe ; do. dorsal integument. 



half cavities usually remain as the dorsal mesentery, while 

 ventrally they are in most instances absorbed. The transverse 

 walls, separating the successive compartments of the body 

 cavity, generally become more or less perforated. 



Chordata. In the Chordata the primitive body cavity is 

 cither directly formed from a pair of alimentary diverticula 

 (Cephalochorda) (fig. 3) or as a pair of spaces in the mesoblastic 

 plates of the two sides of the body (fig. 20). 



As already explained (pp. 294 300) the walls of the dorsal 

 sections of the primitive body cavity soon become separated 

 from those of the ventral, and becoming segmented constitute 

 the muscle plates, while the cavity within them becomes 



I 



