THE ANIMAL KINGDOM 



433 



The mouth opens into a muscular pharynx, usually provided 

 with a radula, a toothed, chitinous ribbon, used for rasping. 

 Attached to the stomach is a large liver. Paired or single 



Fig. 252. — Diagrams of three types of molluscs. A, Gastropod; B, Pelecy- 

 pod (clam), cross section; C, Cephalopod. a, alimentary tract; E, eye; f, foot; 

 g, gills; h, head; m, mantle cavity; s, shell; si, siphon; v, visceral mass; 1, cerebral 

 ganglion; 2, pedal ganglion; 3, visceral ganglion. 



nephridial tubules drain the 'pericardium, which represents the 

 coelom, and open into the mantle cavity. The circulatory sys- 

 tem is composed of a heart with a ventricle and one or two aur- 

 icles, arteries, and veins. The 

 ventricle drives the blood 

 through the arteries to the 

 lacunar spaces in the tissues, 

 from which veins take it to the 

 kidneys and gills, and then to the 

 heart. The blood in the heart 

 is, therefore, always pure. Both 

 dioecious and hermaphroditic 

 forms occur. A veliger-larva 

 stage is common in development 

 (Fig. 253). 



Molluscs are classified as 

 follows : 

 CLASS I. AMPHINEURA. 



Fig. 253. — Veliger larva of Toredo. 

 {After Hatschek.) a, anus; AP, apical 

 plate bearing a cilium; m, mouth; n, 

 nephridial tube; s, shell. (Compare 

 with Fig. 241, A.) 



The nervous system consists of an esophageal ring and four 

 longitudinal nerve cords. Those without a shell are wormlike 

 in appearance. A radula is usually present. All are marine. 



