538 



SIPUNCULOIDEA. 



which give rise to two mesoblastic bands, which do not undergo 

 segmentation. Invaginations of the ectoderm of the animal pole 

 and ventral surface of the embryo give rise to cephalic and ventral 

 plates respectively (Fig. 434), while the remainder of the ectoderm 

 cells grow round these, and form an external envelope for the embryo 

 of the nature of a serous membrane (serosa). Cilia project from the 

 latter through the pores of the vitelline membrane, and are employed 



FIG. 435. Larva of Sipunculus (after Hat- 

 schek). mouth ; Sp apical plate ; A anus; 

 PoW postoral circle of cilia ; N kidney. 



Fio.436. Quite young Sipunculus 

 still without tentacles (after 

 Hatschek). mouth ; A anus; 

 Bs ventral nerve cord ; N neph- 

 ridium (brown tube); G cerebral 

 ganglion ; Bg blood vessel. 



by the embryo in swimming. The cephalic and ventral plates soon 

 grow together. The mesodermal bands soon split into somatic and 

 splanchnic layers, between which is the coelom, and give rise to 

 the rudiments of the two nephridia ; while the oesophagus arises 

 as an invagination of the ectoderm of the antero-ventral region, 

 and a postoral circlet of cilia is formed behind it (Fig. 435). The 

 serous membrane is cast off with the vitelline membrane, and the 



