368 



CH^ETOGNATHA. 



the mesoblast. The anterior paired part becomes partitioned off 

 into a head section and a trunk section (fig. 165 A and B). The 

 former constitutes a pair of distinct cavities (c.pi<] in the head, 

 and the latter two cavities opening freely into the unpaired 

 portion behind. At the junction of the paired cavities with the 

 unpaired cavity are situated the generative organs (gti). The 

 inner wall of each of the paired cavities forms the splanchno- 

 pleuric mesoblast, and the outer wall of the whole the somatic 

 mesoblast. The inner walls of the posterior cavities unite above 

 and below the alimentary tract, and form the dorsal and ventral 

 mesenteries, which divide the body cavity into two compartments 

 in the adult. Before the hatching of the embryo takes place 

 this mesentery is continued backwards so as to divide the 

 primitively unpaired caudal part of the body cavity in the 

 same way. 



From the somatic mesoblast of the trunk is derived the 

 single layer of longitudinal muscles of Sagitta, and part of the 

 epithelioid lining of the body cavity. The anterior termination 

 of the trunk division of the body cavity is marked in the adult 

 by the mesentery dividing into two laminae, which bend outwards 

 to join the body wall. 



The cephalic sec- 

 tion of the body cavity 

 seems to atrophy, and 

 its walls to becomecon- 

 verted into the compli- 

 cated system of muscles 

 present in the head of 

 the adult Sagitta. 



In the presence of 

 a section of the body 

 cavity in the head the 

 embryo of Sagitta re- 

 sembles Lumbricus, 

 Spiders, etc. 



The generative ru- 

 diment of each side 

 divides into an anterior 

 and a posterior part 



FlG. 165. TWO VIEWS OF A LATE EMBRYO OF 



SAGITTA. A. from the dorsal surface. B. from the 

 side. (After Btitschli.) 



m. mouth ; al. alimentary canal ; v.g. ventral 

 ganglion (thickening of epiblast); cp. epiblast ; c.pv. 

 cephalic section of body cavity ; so. somatopleure ; 

 sp. splanchnopleure ; ge. generative organs. 



