THE SNAIL 71 



the apex the whorls end in a small rounded extremity called 

 the nucleus ; this is the oldest part of the shell. A trans- 

 verse section shows that the shell is made up of three layers, 

 similar to those of the shell of Anodonta. In an expanded 

 snail a thick fleshy rim may be seen projecting beyond the 



Fig. 16 



A. Shell of Helix aspersa. col, columella ; z//, points to the position of the 

 umbilicus ; , nucleus ; per^ peristome. B. Dorsal view of Helix aspersa 

 after removal of the shell, ex, excretory organ ; f>\ foot ; F ', hinder part of 

 foot (metapodium) ; /it, heart ; M, mantle with blood-vessels ramifying in it ; 

 M\ thickened border of mantle or collar ; R y rectum ; t, larger or ocular 

 tentacle ; t 1 , smaller tentacle ; ur, ureter. 



aperture of the shell. This is the collar, which secretes the 

 outer layer or periostracum and the middle or prismatic layer 

 of the shell. The inner nacreous layer is formed by the 

 integument covering the visceral hump. Thus the shell in- 

 creases in size by additions to its aperture, and in thickness 

 by additions to the nacreous layer. If the shell is held with 



