270 MANUAL OF ZOOLOGY SECT. 



whole space between the two mantle-lobes, containing the 

 gills, visceral mass, and foot, is called the mantle-cavity. 



Of the muscles the largest and most important are 

 the anterior and posterior adductors (Figs. 147 and 149, #< 

 ad.) p.ad.\ great cylindrical muscles, passing transversely 

 across the body and inserted at either end into the valves of 

 the shell, which are approximated by their contraction. 



The coilome is reduced to a single ovoidal chamber, the 

 pericardium (Figs. 149 and 150, /<:.), lying in the dorsal 

 region of the body, and containing the heart and part of the 

 intestine. In the remainder of the body the space between 

 the ectoderm and the viscera is filled by the muscles and 

 connective tissue. 



The mouth (Fig. 149, mth.) lies in the middle line just 

 below the anterior adductor. On each side of it are two 

 triangular flaps, the internal (1. int. pip.} and external (l.ext. 

 pip.} labial palps ; both are ciliated externally. The mouth 

 leads by a short gullet (Fig. 150, guL} into a large stomach 

 (.$/.), which receives the ducts of a pair of irregular, dark- 

 brown digestive glands (d.gL). The intestine (int.] is given 

 off from the posterior end of the stomach, descends into the 

 visceral mass, where it is coiled upon itself, then ascends 

 parallel to its first portion, turns sharply backwards, and 

 proceeds, as the rectum (ret} through the pericardium 

 where it traverses the ventricle of the heart, and above the 

 posterior adductor, finally discharging by the anus (a.) into 

 the exhalant siphon, or cloaca. The stomach contains, at 

 certain seasons of the year, a gelatinous rod, the crystalline 

 style. 



The gills consist, as we have seen, of two plate-like 

 bodies on each side between the visceral mass and the 

 mantle : we have thus a right and a left outer (Fig. 149, l.ext. 

 gl.\ and a right &&& a left inner gill (l.int.gl.}. Seen from the 



