432 



INVEETEBEATA 



CHAP. 



s.p.g 



vac 



sens 



by the coalescence of the right and left 

 archeuteric folds, and it might almost be 

 regarded as a solid portion of the gut. 



The adult Sagitta exhibits a differen- 

 tiation of the body into head, trunk, and 

 tail. The head swelling is initiated in 

 the embryo by a great anterior dorsal 

 ectodermic proliferation, which later 

 gives rise to the brain or supra- 

 oesophageal ganglion (s.o.g, Fig. 341). 

 It is increased by the formation of a 

 lateral swelling on each side, forming 

 a ridge. These two ridges later unite 

 to form the cephalic hood. Finally, 

 owing to the fact that, as the embryo 

 increases in length, the stomodaeum 

 retains its cavity whilst the rest of the 

 gut becomes compressed to form a 

 vertical solid band, and the cavities of 

 the coelomic sacs also temporarily dis- 

 appear, the breadth of the head region 

 as contrasted with that of the trunk 

 is emphasized, and the head swelling 

 stands out distinctly. This suppression 

 of the cavities seems to be largely due 

 to the circumstance that, whilst the 

 embryo is growing in length and 

 becoming curved, it is still confined 

 within the egg-shell. 



The ventral ganglion makes its 

 appearance as a thickening of the 

 ectoderm, stretching from immediately 

 behind the head to nearly the region 

 where the anus is formed, and taking up 

 two-thirds of the length of the embryo 

 (v.g, Fig. 341). The ectoderm cells become 



FIG. 341. Ventral view. Larva of Sagitta enfiata 

 on the fourth day after hatching, in order to 

 show the origin of the nervous system and the 

 separation of the trunk coelom from the tail 

 coelom. (After Doncaster.) 



a, position of the anus ; l>.c, body coelom ; ent, alimentary 

 canal ; f 1 , / 2 , anterior and posterior fins ; g 1 , mother cell 

 of ovary ; r/ 2 , mother cell of the testes ; hk, masticatory 

 hooks ; in, mouth ; muse, masticatory muscles developed 

 from the head-cavities ; sens, sense-organ (tactile ?) ; sep, 

 transverse septum dividing body coelom from tail coelom ; 

 s.o.g, supra-oesophngeal ganglion ; t.c, trunk coelom ; vac, 

 vacuolated cells (floating tissue) ; v.g, ventral ganglion. 



