XI PHYLUM MOLLUSCA 271 



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. 159 and 161, a, 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 ccelom is reduced to a single ovoidal chamber, the 

 pericardium (Fig. 161, pc) 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. 161, mth) lies in the middle line just 

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

 triangular flaps, the internal (/. int. pip) and external (I. ext. 

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

 leads by a short gullet (Fig. 162, gul) into a large stomach 

 (st), 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. 161, /. ext. 

 gl) and a right and a left inner gill {I. int.gl). Seen from the 



