62 CEYLON PEARL OYSTER REPORT. 



upper part of the adductor. While in front the pericardium is in contact wholly with 

 the viscero-pedal mass, its floor is formed of the wide median communication between 

 the right and left nephridia; posteriorly the pericardial wall is entirely free and 

 coincident with the body-wall and forms the anterior and dorsal boundary of the 

 adductor embayment of the supra-branchial chamber, and laterally its walls are also 

 partially free. 



Dorsally the walls gradually approach, and close to the apex are perforated by the 

 rectum ; so narrowed, however, is the portion of the pericardium above the latter that 

 it appears in sagittal sections rather as a tubular connection uniting the lateral 

 portions of the main pericardial chamber the supra-rectal pericardial arch (P&r.ar., 

 Plate VII., fig. 1). This part of the wall is separated from the dorsal or hinge 

 portion of the body-wall only by some loose connective-tissue. 



Anteriorly there arises on either side from each ventral corner a wide sleeve-shaped 

 prolongation directed forwards, so that the two appear to clasp the viscero-pedal mass. 



These are the two reno-pericardial canals (Rn.per., Plate VII., fig. 1) each of which 

 opens anteriorly by a horizontal slit (Rn.per.', Plate VII., fig. 8) into one of the 

 nephridia close to the external renal orifice. 



The heart largely fills the pericardial space and is clearly distinguished through the 

 thin pericardial wall. It consists of a dorsally situated median ventricle (v., figs. 1 

 and 2) and two lateral auricles (., fig. l) dark-walled and symmetric lying 

 ventral to the ventricle. The auricles are liver-coloured bodies with puckered walls, 

 roughly triangular in form when viewed from behind. The apex of each is attached 

 separately to its respective corner of the base of the ventricle. The two are connected 

 medially by the junction of their inner corners, while thin sheets or partitions of 

 connective-tissue anchor their bases to the floor of the pericardium immediately over 

 the inter-nephridial passage. The walls are largely thickened by the presence of 

 numerous accessory excretory glands the pericardial glands to which is due also 

 their distinctive dark-brown hue (see p. 65 below for details of these glands). 



An efferent blood-vessel from the gills enters each auricle at the outer angle of the 

 base. The auricular cavities inter-communicate through the basal junction and are 

 reduced in capacity by inward projections of the walls. 



The lips of the auriculo- ventricular apertures project inwards and form simple yet 

 effective valves preventing the reflux of blood into the auricles during the ventricular 

 systole. The ventricle is elongated, of a pale yellowish-white tint ; the walls are 

 thick and muscular, and the cavity is further reduced by numerous muscular trabecular 

 crossing in various directions. As Ghobben first pointed out, this ventricle does not 

 surround the rectum, as is so usual in Lamellibranchs, but its dorsal extremity is 

 intimately fused with the lower surface of the rectum. The muscle fibres in the wall 

 of the heart are distinctly striated (Plate IX., fig. 12). 



Anterioi'ly and posteriorly the dorsal ends of the ventricle pass into the anterior 

 and posterior aortie respectively. The latter (Ao.j)., Plate VII., fig. 2), the smaller of 



