MOLLUS C A — CEPH ALOPODS 257 



able to cut off the leg of a chicken as if with a pair of shears. 

 The upper jaw fits closely inside the lower so that their action 

 in biting is like that of a steel shear where two heavy blades 

 having sharp square corners move past each other. The jaws 

 form the main part of the buccal mass, leaving a comparatively 

 small buccal cavity. As in the Gastropoda, Scaphopoda, and 

 Amphineura, the floor of the buccal cavity bears a radula, set 

 with pointed teeth, also a large fleshy projection called the 

 tongue. The walls of the intestine are extremely thin and have 

 an abundant supply of blood vessels which thus are in position 

 to gather up the products of digestion from the digestive canal 

 and carry them throughout the body. 



The heart consists of a large, muscular ventricle into which 

 open four branchial veins, one from each of the four gills. The 

 blood passes from the ventricle in five arteries, the divisions of 

 which penetrate all parts of the body. These arteries finally 

 subdivide into capillaries and from these the blood passes 

 into the sinuses, — extensive blood spaces in the various 

 tissues. 



Four nephridia or kidneys discharge their waste products into 

 the mantle cavity. It is seen, then, that excretion of the body 

 wastes is effected through the kidneys and gills which purify the 

 blood, and by means of direct discharge by the anus through 

 which passes the unused products of digestion. 



The nervous system possesses no distinct ganglia, the central 

 nervous system being represented by a very thick nerve collar 

 surrounding the oesophagus. From this collar pass nerves to 

 the various organs of the body. 



Sense organs. — The eyes (Fig. 112, e.) are very large, nearly 

 an inch in diameter, but extremely simple, each consisting essen- 

 tially of a saucer with convex bottom attached to the side of the 

 head by a stalk and with the top covered by a disk perforated by 

 a central aperture ; this saucer is lined by the retina and possesses 

 no lens or vitreous humor. It is filled by the sea water, which has 

 free access through the aperture at the top. Thus the naked 

 s 



