546 THE INVERTEBRATA 



its edge (Fig. 377). The outer shell layer, periostracum, formed of 

 horny conchiolin, is first produced in a groove and then the prismatic 



ap.o. 



?^-prt. 



nres. 



-^mes. 



Fig. 374. 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. em- 

 bryonic muscle cells; prt. prototroch; sh. shell; st. stomach; t.t. telotroch; 

 D and V dorsal and ventral. 



/ , vm. 



an.- 



vm. 



an. 



sh. 



Fig. 375. Veliger larvae. A, 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., 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. 



layer, largely consisting of calcite or arragonite, is secreted underneath 

 it by the cells of the thickened edge. The innermost nacreous layer 

 (also mostly CaCOg) is, however, formed by the cells of the whole of 



