vni ENTERIC CANAL 401 



The ccelome is reduced to a single ovoidal chamber, the 

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

 of the body and containing the heart and part of the intes- 

 tine : it is lined by ccelomic epithelium, and does not corre- 

 spond with the pericardial sinus of the crayfish, which is 

 a blood-space (p. 376). In the remainder of the body the 

 space between the epiderm and the viscera is filled by 

 the muscles and connective-tissue. 



The mouth (Fig. 101, mtJi) 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 (st\ which receives the ducts of a pair of 

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

 (tnt)\ which is lined by a ciliated epithelium, 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 (rct\ 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 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 con- 

 tains 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 either side between the visceral mass and the 

 mantle : we have thus a right and a hft outer, and a right 



PRACT. ZOOL. D D 



