MOLLUSCA LAMELLIBRANCHIATA. 109 



times very small. In the oyster tribe it is altogether 

 absent. 



The animal is provided with muscles, by which it is 

 enabled to protrude or retract the foot at will. These 

 retractor muscles of the foot leave certain markings in 

 the interior of the shell, which are called the pedal 

 impressions. 



There are other " impressions " in the interior of the 

 shell, of which the principal are the pallial line, and 

 the pallial sinus. 



The pallial (Lat. pallium, a cloak) line marks the 

 impression made by the margin of the mantle. 



The pallial sinus (Lat. sinus, a bay), is an indentation 

 in the pallial line, found in those species that have re- 

 tractile breathing siphons. The depth of the indentation 

 corresponds with the length of the siphons. 



151. Digestion. The Lamellibranchiata are destitute 

 of a distinct head. The mouth is placed in the front 

 part of the body, and is furnished with one or two pairs 

 of membranous organs called palpi. There are no teeth. 

 The mouth is followed by a gullet which conducts to a 

 stomach and convoluted intestine. The intestine passes 

 through the heart, and terminates in an anus, placed 

 near the aperture of the breathing siphon. There is a 

 large liver, but no salivary glands. Their food consists 

 principally of small animalcules. 



152. Circulation. There is a well developed heart, 

 consisting often of three chambers two auricles and one 

 ventricle. Sometimes there is only one auricle and one 

 ventricle, as in the oyster. In a few instances there are 

 two hearts, placed in different parts of the body, each 

 composed of an auricle and a ventricle. There are arteries 

 which convey the blood from the ventricle through tho 

 body, and veins which carry it to the gills, where it is 

 rerated. From these it is brought back to the auricle or 

 auricles. 



153. Respiration. The organs of respiration consist 

 of a pair of plate-like gills on each side of the body. 



