GENERAL NOTES ON MOLLUSCS. 



365 



branchs, and in Dentalium, it is modified as a conical boring 

 organ. 



The mantle is another important Molluscan structure, 

 and as it secretes the shell, the shape of the latter is of 

 course determined by it. Primitively the mantle is repre- 

 sented by a uniform downgrowth of skin from the dorsal 

 surface, surrounding the ventral foot, and secreting a dorsal 

 cap-shaped shell. Such a simple condition occurs in the 

 limpet. In the Lamellibranchs, with the lateral flattening 

 of the body, the mantle becomes divided into right and left 

 halves, and the shell becomes two-valved. In most Lamelli- 

 branchs the mantle is prolonged into two tubes or siphons, 

 through which the water of respiration enters and leaves the 



■Mff 



Fig. 159. — Bivalve (Panopcea norvegica), showing siphons. 

 e., Exhalant aperture ; /., inhalant aperture. 



mantle cavity. A similar but unpaired siphon is found in 

 many Gasteropods. In Scaphopoda the mantle folds fuse 

 ventrally to form a continuous tube. In most Gasteropods 

 the mantle skirt is retained, and secretes a spiral shell, as 

 well as enclosing a space in which the gills lie ; in some, 

 both mantle and shell are absent. In 'the snail and its allies 

 (Pulmonata), the mantle fuses with the body-wall and forms 

 the pulmonary chamber, which opens to the anterior by a 

 small aperture. In Cephalopoda the mantle skirt is well 

 developed and muscular, and, besides sheltering the gills, is 

 of much importance in locomotion. 



Typically the Molluscaare bilaterally symmetrical animals, 

 and this symmetry is marked in the Amphineura, the 

 Lamellibranchiata, and occurs to a less extent in the 

 Cephalopoda (cf. the unpaired genital organs). In the 



