MOLLUSCA 



475 



the same habits, browsing on small algae and returning after ex- 

 cursions to a centrally situated home. In dorsal view there are seen 

 the eight shell plates which articulate with one another and allow the 

 animal to roll up like a wood louse. Each plate is composed of two 

 layers, the upper or tegmentum and lower or articulamentum. Both are 



nac. 



prS7tiJ 



yCx.ma. ov. 



ma- 



Fig. 352. 



prsm: -^prst: 



Fig. 353- 



Fig. 352. Ventral view of Chiton to show external and internal bilateral sym- 

 metry. Mantle cavity finely stippled, the divisions of the coelom, shown 

 above the foot, coarsely stippled, an. anus ; ct. ctenidia ; e.k.a. external kidney 

 aperture; i.k.a. internal kidney aperture; gen. gonad; g.a. genital aperture; 

 k. kidney; M. mouth; pern, pericardium. 



Fig. 353. Vertical section through the edge of the mantle of Mytilus. ex. ma. 

 external, in.ma. internal surface of mantle ; gl. gland cells ; nac. nacreous layer; 

 prsm. prismatic layer of shell; prsm.' prisms arising at external border; prst. 

 periostracum ; prst.' periostracum arising in a fold of the mantle ; ov. ova in 

 the mantle tissue. After Field. 



calcareous, but the tegmentum is traversed by parallel canals which 

 end on the surface in remarkable sense organs ; some of these have the 

 structure of eyes. There is little evidence, however, that they are 

 sensitive to light. Others are sense organs of quite unknown function 



