MOLLUSC A 279 



Aspidobranchia have feather-shaped (double) gills which are free at the 

 tips. The Ctenobranchia have a single comb-shaped (single) gill. The 

 Heteropoda are pelagic. The foot forms a flat fin. The visceral mass is 

 small and the shell poorly developed or wanting. 



602. The Euthyneura have the visceral nerves parallel. They are 

 hermaphrodytic. The Opisthobranchia are marine forms with the gills 

 usually behind the heart. The Pulmonata are chiefly terrestrial and 

 fresh water snails without gills. The mantle cavity serves as a lung. In 

 the slugs the shell is greatly reduced or wanting. 



603. Order 2. The Solenoconcha have a horn-shaped shell and body 

 and a cylindrical foot. The group is small and the animals are also small. 

 They are marine and live in the mud of the bottom. 



FIG. 165. A slug, Limax. (From Galloway, after Binney's Gould.) 



604. Order 3. The Lamellibranchiata are compressed laterally. The 

 head is rudimentary. The mantle is large and double, right and left. 

 The shell is also double and the two valves are connected by a dorsal 

 ligament. The foot is usually wedge shaped. There are two pairs of 

 plate-like gills. The animal is usually bilaterally symmetrical but there 

 may be considerable deviation from this rule. 



605. The shell is secreted by the mantle and is composed of three 

 layers. On the surface is a thin layer of a horny cuticula (periostracum) 

 which is formed by the extreme edge of the mantle. The hinge ligament 

 is of the same substance but forms a very thick layer. The hinge is elastic 

 and causes the shell to gape when the adductor muscles are relaxed. 

 Beneath the cuticula there is a thick layer of calcium carbonate deposited 

 in a matrix of organic matter (conchiolin). The limey portion of the shell 

 consists of two layers, an outer "prismatic" layer and an inner layer of 

 "mother of pearl." The prismatic layer is so called because of the colum- 

 nar or prismatic arrangement of the substance. The prisms stand perpen- 

 dicular to the surface. The "mother of pearl " is in layers parallel to the 



