12 STUDIES IN BIOLOGY. 



the stomach along the floor of the pericardial cavity until it is near 

 the anterior side of the posterior adductor. This part of the intestine 

 I call the direct intestine, as distinguished from the recurrent part of 

 the intestine that succeeds it. Near the posterior adductor the direct 

 intestine turns sharply on itself in a horizontal plane and to the right 

 side (re int, Fig. 13), becoming the recurrent intestine. The recurrent 

 intestine runs forward on the right side of the pyloric caecum and 

 pericardial artery to about the middle of the direct intestine, when it 

 crosses upwards over the direct intestine, pyloric caecum, and rectum 

 (rect, Figs. 13 and 14), to the left side of the stomach, passing "be- 

 tween the middle retractors of the byssus. It can now be seen from 

 the left side among the hepatic caeca as it passes down obliquely to 

 the anterior end of the stomach. Near the anterior end of the 

 stomach it turns down and back, being now ventral to the greater 

 part of the liver. It rises gradually as it returns until between the 

 middle retractors of the byssus (Fig. 1), when it is immediately below 

 the recurrent intestine. Then, as the rectum, it passes directly back 

 through the pericardial cavity (Fig. 1), penetrating the ventricle. 

 Leaving the pericardial cavity it goes down the upper surface of the 

 posterior adductor, ending in the anus on the hinder upper side of 

 that muscle (Fig. 1). 



The pyloric caecum, containing the crystalline style, leaves the 

 stomach just above the intestine. It keeps on the upper side of the 

 direct intestine through the whole length of the latter, being very 

 closely applied to it, and almost united with it, yet forming two 

 separable tubes (p c, Figs. 13 and 14). When the direct intestine 

 ends and the recurrent intestine begins, the pyloric caecum continues 

 its course backward alone, now, however, leaving the median line and 

 inclining to the left. It passes above the posterior adductor to the 

 left of the rectum, and enters the left mantle-lobe (p c, Fig. 13), 

 gradually tapering as it proceeds. In the left mantle-lobe its course 

 is a little below the thickening of the mantle edge, and the ridge 

 caused by it is easily seen when the anal membranes are cut away 

 (Fig. 1). It forms a curve concentric with that of the mantle edge, 

 entering the thickening of the mantle edge above the posterior junc- 

 tion of the mantle-lobes. It passes the junction, and gradually tapers 

 until it can no longer be followed, having now a course similar to that 

 of the marginal or circumpallial nerve. 



There is considerable difference between M. latus and edulis as to 

 the disposition of the pyloric caecum and the intestine ; and, as I shall 

 have occasion to criticise Sabatier's (7) account of the alimentary 

 canal, it may be well to give his account of the arrangements in M. 



