378 



INVERTEBKATA 



CHAP. 



(y.g, Fig. 306) are shed into the coelom and are there taken up by 

 amoebocytes and absorbed. In this way, gradually, the whole of 

 the prototroch is disposed of. During this process the larva takes 

 on a cylindrical form with a diminishing ring-shaped swelling in 

 front ; this swelling is the disappearing prototroch. 



A nerve strand runs from the cerebral ganglion to the apical 

 sense-organ, and from this ganglion originate a pair of muscle cells 



mes 

 eel 



end 



mtr 



v.n.c 



y-9 



end 



coe 



FIG. 306. Nearly sagittal sections through metamorphosing trochophores of Phascolnsoir.fi. 

 A, sagittal section to one side of the middle line of the larva of Phascolosoma gouldii, 

 about fifty-seven hours old, to show the segments of the mesodermal band. (After two 

 figures by Gerould combined. ) B, sagittal section nearly median of the larva of 1'has- 

 colosoma vulgare, about forty-eight hours old. (Al'ter Gerould.) 



(i, anus; up, apical plate; coe, coelomic cavity; end, endodermic tube; mesi, meift, wes :( , .segments 

 of the mesodermic band ; mes.ect, point of origin of the mesectoderm ; mtr, metatroch ; n.coll, nerve 

 collar ; pr.c, prae-oral circle of cilia ; jitr, degenerate prototrochal cells ; ret.acc, accessory retractors ; 

 ret.d, dorsal retractor muscles formed from ectoderm; ret. v, ventral retractor muscles formed from 

 ectoderm; s.o.y, supra-oesophageal ganglion ; storm, stomodaeum ; r.n.i; ventral nerve cord ; y.g, yolk- 

 granules, remains of prototrochal cells. 



which run backwards and are inserted into the dorso-lateral region 

 of the skin behind the prototroch. At the sides of the ventral 

 nerve plate there are situated two series of clusters of muscle cells. 

 The more ventral of these run backwards towards the posterior 

 insertion of the retractors ; the more dorsal extend forwards to the 

 region between prototroch and post-oral circlet. 



The invagination of the anterior region of the body, so as to form 



