PELECYPODA 73 



The circulatory system consists of a dorsal heart of one 

 ventricle and one or more auricles, enclosed in a pericardium. 

 Aortas carry the blood from the ventricle to different parts of the 

 body, but the blood-vascular system is not entirely closed. 



Respiration is carried on through the body wall in a few 

 Mollusca, Ijut most of them breathe through gills or lungs. 



The nervous system is characterized by three pairs of gangha 

 which are joined ])y connective nerve cords. The cerebral 

 ganglia are situated dorsal to the esophagus and supply the 

 tentacles and eyes. The pedal ganglia lie ventral to the mouth 

 and supply the foot and otocysts. The visceral ganglia, also 

 ventral, but farther back, supply the body, the mantle, and the 

 so-called " osphradia," or olfactory organs. Some moUusks 

 lack special sense organs. 



Locomotion is accomplished by the single so-called " foot," 

 a muscular plowshare-shaped thickening of the body. 



Multiplication. — The Mollusca may be sexually separate or 

 hermaphroditic. 



This l^ranch includes some very valuable food animals for man, 

 as clams and oysters. Other examples are snails, slugs, scallops, 

 cuttle-fishes, squids, and fresh-water mussels. 



CLASS L PELECYPODA 



This class is called by various names by chfferent zoologists, 

 depending upon the character taken for the basis of classifica- 

 tion — as Aglossa (without a tongue), Acephala (without a 

 head), Bivalva (of two valves), Pelecyp'oda (hatchet-footed),^ 

 Lamellibranchiata (leaf-like gilled). We may then characterize 

 this class as the hatchetciooted, headless, tongueless, bivalved, 

 leaf-like gilled mollusks. Mussels, clams, and oysters are com- 

 mon examples of this class. 



The body is soft, unsegmented, and is modified into the large 

 " foot " used for locomotion. The mantle, a great fold of skin, 

 covers the body, one lobe over each side. Between the mantle 

 lobes and the body are the four large leaf-like gills. The labial 

 palpi are the small leaf-like structures anterior to the gills, and 

 lead into the mouth. This organ secretes the shell. 



Food consists of small organisms which the water carries 

 into the mantle cavity and to the ciliated labial palpi, which 



