THE AMPHINEURA 



43 



from head to anus. The breadth of the foot is inversely pro- 

 portional to the width of the lower edge of the mantle : it is broad 

 in most of the Polyplacophora, but narrow in the Cryptoplacidae. 



Extending all round the foot, between it and the mantle on the 

 ventral side, is the pallial groove, in which lie the gills. In the 

 more primitive Chitons mucous thickenings extend into the groove, 



Fio. 24. 



Transverse section of the lateral portion of the pallial teguments of Chiton. I, tegmentum ; 

 II, articulainentum ; III, pallial epithelium under the shell ; IV, epithelium of the margin of 

 the mantle; V, spicula ; VI, cuticle of the mantle-margin; Via, periostracum ; VII, megal- 

 aesthetes ; VIII, micraesthetes. (After Blumrich.) 



reaching from the anus to its upper corner, or to the foot, or even 

 to the inner wall of the mantle. 



II. ANATOMY. 



1. The Alimentary Canal extends from one extremity of the body 

 to the other. The mouth leads into the buccal cavity, on the ventral 

 wall of which opens the radular caecum. The radula is long and 

 reaches nearly as far back as the stomach. Each radular row 

 includes seventeen teeth of various shape (Figs. 2, A ; 74, E). The 

 three central teeth are simple : on each side of them is a large 



