viii ENTERIC CANAL 401 



The ccelome is reduced to a single ovoidal chamber, the 

 pericardium (Figs. 101 and 103, pc), lying in the dorsal 

 region of the body and containing the heart and part of 

 the intestine : it is lined by coelomic epithelium, and 

 does not correspond with the pericardial sinus of the 

 crayfish, which is a blood-space (p. 376). In the remain- 

 der of the body the space between the epiderm and the 

 viscera is filled by the muscles and connective-tissue. 



The mouth (Fig. 101, mth) lies in the middle line just 

 below the anterior adductor. On either 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 (gut) into a large stomach (s/), which receives 

 the ducts of a pair of irregular, dark brown, digestive 

 glands (d. gl). The intestine (int), which is lined by a 

 ciliated epithelium, comes 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 tr^e 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 wall of the rectum is 

 produced into a longitudinal ridge, or typhlosole (Figs. 

 101 and 103, ty), and two similar ridges begin in the 

 stomach and are continued into the first portion of the 

 intestine. 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, which represent a single ctenidium (see Fig. 1036), 

 on either side between the visceral mass and the mantle : 

 we have thus right outer and inner gill-lamince and left 



