MOLLUSCA. 



241 



280. General Survey. The more commonly known forms 

 are easily recognizable by the hard calcareous shell which 

 protects the soft unsegmented body within. The shell may be 

 in two sub-equal valves, right and left, or may be in one piece, 

 in which case it is usually coiled or spiral (Fig. 108). The 

 bivalved types are able to open and close the shell after the 

 manner of a box, and the soft parts are further capable of 



FIG. 1 08. 



c 



FIG. 108. Helix. A, an empty shell in section from apex to base, a, apex of shell; 

 an., anus; ap., aperture of shell; c, columella or axis of shell; e, eyestalk; /, foot; 

 I, lip of shell; m, edge of mantle, which secretes the shell; r.a., respiratory aperture; 

 s, suture, between the whorls; t, tentacles. B, the relation of the animal to the shell 

 when extended. 



Questions on the figures What suggestions of bilateral symmetry 

 are shown by the snail? Where does growth occur in the shell? What 

 are the functions of the tentacles? What is the function of the edge of 

 the mantle called the "collar" (m) ? 



protrusion from the partly opened shell. This latter power 

 is much more pronounced in the univalved types (e. g., snail). 

 The fundamental bilateral symmetry is obscured in the more 

 sluggish forms, but is very decided in such active animals as 

 the squid and some of the bivalves. 



One of the most interesting points of difference among the 

 members of the group is the degree of development of the 

 17 



