MOLLUSCA 



473 



a fashion identical with that described for the anneUd. In the diagram 

 given here for Patella , we see the completion of gastrulation and the ap- 

 pearance of the ciliated rings of the trochosphere(Fig. 350 A) ; also the 



ap.o. 



Fig. 350. Patella coerulea. A, Trochosphere larva, sagittal section. B, Early 

 veliger larva, sagittal section. C, Veliger larva, frontal section to show meso- 

 derm bands. After Patten, ap.o. apical organ; end. endoderm; F. foot; 

 int. intestine; M. mouth; mes. mesoblast pole cell and derivatives; m. 

 embryonic muscle CQ\\s;prt. prototroch; sh. shell; st. stomach; t.t. telotroch. 



-dig.gl. 



sh. 



Fig. 351. Veliger larvae. Ay Ostrea edulis, side view. After Yonge. Ciliary 

 currents shown by arrows. Suspended material is thrown by the action of 

 the large cilia of the velum on to the ciliated tract, ct., imbedded in mucus 

 and carried to the mouth, M., then through the oesophagus into the stomach, 

 St. The style, shown by stippling, projects from the style sac, s.s.y in which it 

 rotates; many particles are imbedded in this. After leaving the stomach the 

 material passes through the coiled intestine (dotted) and by the rectum, rm., 

 out into the mantle cavity, ma.c. Other letters: an. anus; a.m. adductor 

 muscle; dig.gl. digestive gland; F. foot; sh. shell; vm. velum. B, Dreissensia, 

 ventral view. After Meisenheimer. 



single large cell which gives rise to the mesoderm. Then in Fig. 350 B 

 we see the early veliger with an internal organization similar to 

 the annelid, with apical organ, larval nephridia ^nd prototroch. The 



