DESCRIPTION 



479 



visceral mass; in other mollusks, on the other hand, the mantle is a 

 single fold covering- the visceral mass on all sides as a thimble covers 

 the linger. 



The outer surface of the mantle secretes the shell, which may be 

 regarded as the cutieula and is usually charged with calcium carbonate, 

 while the space between the mantle and the visceral mass is called the 

 mantle cavity (3). This space is important in that it contains the 

 gills, and that the digestive, excretory, and reproductive organs usually 

 empty their products into it. A mantle is wanting in many mollusks, 

 which are also without a shell. Other mollusks which have a mantle 

 may lack the shell; in many slugs and also in the squids the shell is 

 buried in a pocket of the mantle and is not on the surface. The shell 

 thus takes the form of the mantle, and may be bivalved or univaived, 



Fig. 741 



Fig. 742 



Fig. 741 — ^^Diagrammatic cross sections of gastropods. A, a prosobranch with 

 epipodia ; B, an opisthobranch with parapodia (Lang). 1, shell; 2, visceral mass; 

 3, foot ; 4, gill ; 5, epipodium ; 6, parapodium. Fig. 742 — The mouth and pharynx 

 with the radula of a snail (Lang). 1, pharynx; 2, radula ; 3, jaw; 4, mouth; 

 5, tongue ; 6, opening of salivary glands ; 7, oesophagus. 



and spiral or cone-shaped. The shell is usually composed of three 

 layers, the inner mother-of-pearl layer which is secreted by the general 

 surface of the mantle, and the middle columnar and outer periostracal 

 layers which are secreted by its lower margin. 



The foot is flat and used for creeping by the chitons, gastropods, 

 and a few pelecypods, this probably being its primitive form and use. 

 It may have a variety of forms in otber mollusks and the following 

 divisions may in certain cases be distinguished in it: the propodium or 

 forward and the metapodium or hinder portion, the parapodia or lobe- 

 like extensions of the sole (Fig. 741,6), and the epipodium, a ridge 

 along the side (Fig. 741,5). 



Internal Structure. — Correlated w^ith the compactness of the visceral 

 mass is the. small extent of the coelom, which is usually confined to the 

 pericardial space and the cavities of the gonads. The mouth opens 

 into a muscular pharynx (Fig. 742) in all mollusks except pelecypods, 



