THE PHYLUM MOLLUSCA 



245 



venous sinuses, spaces among the various organs that are difficult to 

 observe. Most of tlie returning blood passes through the excretory organs 

 and then either directly to the auricles or through the gills to the 

 auricles. The amount passing through the gills determines the amount 

 of freshly oxygenated blood returning to the heart, and varies from 

 mollusc to mollusc according to its level of activity. 



The excretory organs are a single pair of nephridia, intimately as- 

 sociated with the circulatory system. In each a large glandular region is 

 bathed in blood, and from this a tubule opens externally at a nephridio- 

 pore. In many molluscs the inner ends of these tubules open into the 

 pericardial cavity. 



The phylum includes three large classes having species of economic 

 importance and two additional small classes (Fig. 14.2). The classes are: 







~^s 



Amphmcura 



Felecy po<i^% 



Figure 14.2. Classes of the phylum Mollusca. Letters indicate structures that are 

 part of the shell (S), mantle (M), and foot (F). (Gastropoda after KHne in Curtis and 

 Guthrie, 1938; Scaphopoda, original; others after Lankester, 1906.) 



