ANATOMY OF THE PEARL OYSTER. 57 



fold, the typhlosole (Plate VI., figs. 3 and 1 I, Ty.) projecting inwards. At its start, 

 tins typhlosole projects from the anterior wall, but almost at once curves over to the 

 posterior side of the tube, thence running vertically upwards without further change 

 of course. Longitudinal and somewhat oblicpue furrows channel the surface, and as it 

 expands greatly above the line of attachment to the intestinal wall, its bulk largely 

 fills the cavity of the intestine. In transverse section, the lumen is seen to be reduced 

 to an attenuate long-horned crescent (Plate VI., fig. 5). As it approaches the level 

 of the floor of the stomach, the typhlosole thins down rapidly to a low ridge, and the 

 intestine itself then curves posteriorly in the direction of the heart (Plate VI., fig. 1). 

 This change in direction and thinning down of the typhlosole indicate the commence- 

 ment of the rectum (Int.3, Plate VI., fig. 1), which is not marked by any other 

 definite sign. 



From the right-angled curve made by the intestine posterior to the stomach, the 

 rectum runs posteriorly, through the upper part of the pericardium. Beyond this it 

 begins to curve ventrally, and passes round the posterior aspect of the adductor 

 muscle in the median line, ending in an erectile ear-like process bearing the anus 

 (Plate VI., fig. 1), and situated opposite the exhalent orifice of the mantle. 



The rectal typhlosole, though well-marked as a semi-cylindrical ridge (Plate VI., 

 fig. 12, Ty.), never rises much above the semi-diameter of the tube. It runs along 

 the intestinal floor while in the cardiac region, and when the rectum courses behind 

 the adductor muscle it becomes a median fold on the anterior wall. 



Where the first portion of the rectum passes through the narrow upper part of the 

 pericardium, it has the ventricle attached to its lower surface (Plate VI., fig. 1) with 

 an anterior aorta running forwards and upwards to the left, and a posterior aorta 

 passing backwards posteriorly and ventrally to pursue a course parallel with the 

 rectum (see also Plate VII., fig. 2). From the point where it passes from the cardiac 

 region, the rectum and the accompanying aorta are overlaid with a considerable 

 thickness of spongy lacunar tissue (Plate IX., fig. 6). The bulk thus attained renders 

 the course of this division of the intestine conspicuous as a massive semi-cylindrical 

 vertical ridge descending the posterior surface of the adductor in the median line. 

 This rectal ridge (JR.r., Plate VI., fig. 15) is frequently pigmented with splashes of 

 black and opaque white pigments occasionally it is suffused more or less extensively 

 with orange. Figs. 6 and 7 on Plate IX. show the range in height and shape of the 

 rectal ridge, and fig. 8 gives the character of the tall ciliated epithelium of the rectum. 

 The anal process (An./., Plate VI., fig. 3) is a comparatively large, slightly curved, 

 erectile, ear-shaped organ facing ventrally. It stands out at right angles to the last 

 section of the rectum, and the tip is directed posteriorly, while the margins tend to 

 be somewhat in-curved. The anal ajuerture (an.) is situated at the base, on the 

 ventral aspect. Fa3cal matter is expelled periodically in string-like masses, which, 

 caught up by the steady exhalent current, are swept out through the mantle orifice 

 and carried some little distance clear of the animal (Plate VI., fig. 14, Pel.). 



I 



