XXVIII DIGESTIVE ORGANS 355 



are called the anterior (a. r) and posterior {p. r) retractors. 

 A third muscle {pre) is inserted into the shell close 

 to the anterior adductor, and has its fibres spread fan-wise 

 over the visceral mass which it serves to compress, thus 

 forcing out the foot and acting as a protractor of that organ. 

 The substance of the foot itself consists of a complex mass 

 of fibres, the ifitrifisic muscles of the foot, many of which 

 also act as protractors. Lastly, all along the border of the 

 mantle is a row of delicate pallial muscles (Fig. 92, b, //. 

 m), which, by their insertion into the shell, give rise to the 

 pallial line already seen. 



The ccBlome is reduced to a ' single ovoidal chamber, the 

 pericardium (Fig. 93, Cxi ; Fig. 94, pc), lying in the dorsal 

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

 intestine: it is lined by coelomic epithelium {Ccel. Epthm), 

 and does not correspond with the pericardial sinus of th'* 

 crayfish, which is a blood-space. 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. 94, mth) lies in the middle line, just 

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

 triangular flaps, the internal and external labial palps ; the 

 external palps unite with one another in front of the mouth, 

 forming an upper lip ; the internal are similarly united 

 behind the mouth, forming a lower lip : both are ciliated 

 externally. The mouth leads by a short gullet (Fig. 94, 

 gul) into a large stomach (st), which receives the ducts of a 

 pair of irregular, dark-brown digestive glands (d.gl). The 

 intestine (int) goes 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, 



A A 2 



