548 THE INVERTEBRATA 



occupies the whole ventral face of the body ; mantle containing eight 

 transverse calcareous plates as well as spicules ; in the mantle groove 

 which runs entirely round the body there is a more or less complete 



prst,^ 



prsm. I 



nac. 

 I /ex.ma. 



I 



-in.ma' 



prsm: -^prst: 



Fig. 376. 



Fig. 377. 



Fig. 376. 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 ; pcm. pericardium. 



Fig. 377. Vertical section through the edge of the mantle of Mytilus. ex.ma. 

 external, in.ma. internal surface of mantle;^/, 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. 



row of ctenidium-like gills on each side, Chiton (Figs. 373 B, 376), 

 Craspedochilus . 



Aplacophora. Worm-like Amphineura in which the foot is absent 

 or represented by a median ridge in a ventral groove and the mantle 

 correspondingly enlarged. No shell plates but spicules only. Mantle 



