284 



THE INVERTEBRATA 



the rudiment of a large part of the trunk of the aduh worm. It will 

 form ventral nerve cord, chaetal sacs and the ventral and lateral ecto- 

 derm of the trunk. The larval gut opens by a mouth in the equatorial 

 region and consists of an ectodermal oesophagus (stomodaeum) open- 

 ing into the endodermal stomach and an ectodermal hind gut opening 

 to the exterior by an anus. The cavity between the ectoderm and the 

 gut (blastocoele) is spacious and traversed by the pseudopodia-like 

 processes of the mesenchyme cells, larval muscles and nerves, and 

 also contains the two larval nephridia, each of which is composed of 

 two hollow cells placed end to end, one of which contains a "flame'* 

 of cilia. They are descended from the first quartette of micromeres 

 and sink in from the surface. 



mesc 



^-prt. 



DCS. 



an.v.^ 



Fig. 197. Trochosphere larva of £'M/)owaiw^. Side view. After Shearer, ap.o. 

 apical organ; e. eye; prt. preoral ciliated ring; hk. "head kidney", larval 

 nephridium ; otc. otocyst ; mes. mesodermal band ; an. anus ; an.v. anal vesicle ; 

 bl.c. blastocoele ; mesc. mesenchyme ; oes. oesophagus ; st. stomach. 



The trochosphere drifts hither and thither in the sea, swimming 

 feebly by the action of the ciha of the prototroch and sometimes also 

 by secondary postoral rings of cilia (e.g. metatroch formed from cells 

 of the third quartette). During this pelagic existence the rudiments of 

 the adult worm continue their development — which is best traced in 

 Polygordius — the apical organ develops into the prostomium of the 

 adult with brain, tentacles and eyes, while the trunk rudiment formed 

 by the proliferation of the ventral plate and the mesoblast cell grows 

 backwards as an ever-lengthening cyHndrical process containing the 

 end gut. In the ectoderm of this is developed ventrally the rudiment 

 of the ventral nervous system, while to the sides of this and internally 

 are the mesodermal strips (derived from the single cell d^), which 



