MOLLUSCA 



251 



tions passing from the dorsal to the ventral margin (see dissec- 

 tion of clam). The walls of the gills contain blood vessels, and 

 the water, assisted by the action of the cilia, circulates over and 

 through the gills. In the cephalopods and aquatic gasteropods 

 the gills occur as tufts of filaments, which may or may not be 

 covered by the mantle. In addition to these special organs the 

 mantle and the soft body surface assist in respiration. (For 

 figures of the gill structure in the clam see Parker and Haswell's 

 Text-book of Zoology, Vol. I, Fig. 669.) 



292. Circulation. — There is usually a well-developed circu- 

 lation of the blood, but a portion of it occurs through irregular 

 spaces devoid of walls proper. The organs consist of a contrac- 

 tile heart usually with undivided ventricle and a single auricle 



Fig. 113. 

 heart ■ 



tissues 



tissues 



Fig. 113. [Diagram 'showing the general course of the circulation in moUusks. Compare with 

 Fig. 112, which shows the organs more nearly in their relative position. 



Question on the flgure. — Why does the blood which passes to the mantle not 

 need to pass the gills before returning to the heart? 



(gasteropods),! or one pair of auricles (lameUibranchs, squid), 

 or two pairs (Nautilus). Definite arteries pass both forward 

 and backward from the ventricle. The blood passes from the 

 ventricle to the tissues of the body, whence it gathers into 

 venous spaces and passes into the kidneys and the gills by way of 

 a principal vein. From the gills it finds its way to, the auricles. 

 In lameUibranchs the blood which goes from the ventricle to 

 the mantle is aerated and returns directly to the auricle. In 

 some cephalopods there are branchial hearts near the gills to 



