64 CEYLON PEARL OYSTER REPORT. 



the lacunae irregular 1 spaces between the tissues whence it drains into larger 

 cavities, the venous sinuses, which conduct directly to the heart (from the mantle) or 

 indirectly by the intermediation of the gills. 



The blood is colourless, and contains nucleated corpuscles, which are shown in some 

 of the figures of the blood spaces in the gills (Plate VIIL, figs. 11, 13). 



THE EXCRETORY SYSTEM. 



The renal excretory system consists of the paired nephridia and possibly of the 

 numerous small pericardial glands projecting from the walls of the auricles. The 

 nephridia consist of two large symmetrical pouch-like sacs lying one on either side of 

 the hinder half of the viscero-pedal mass. Each opens into the pericardium by a 

 wide duct and to the exterior by a minute pore, and they intercommunicate by a 

 wide channel beneath the auricles. In outline each is roughly triangular, the apex 

 passing into the channel under the auricles, while the elongated base looks outwards 

 and forwards, coinciding with the base of the anterior third of the gill of that side, 

 and thus conforming to the inclination of the gill. 



The outer wall of the nephridium (Neph., Plate VII., fig. 8) is thin and mem- 

 branous ; it is fused with the body-wall, as is also the most anterior portion of the 

 inner wall, namely, that strip extending from the base of the gill to the viscero-pedal 

 mass ; from this line it runs back, overlying and in contact with the hinder part of 

 the gonad, gradually narrowing as it approaches the auricle. 



The external renal aperture (Iin.o., Plate VII., fig. 8) is a minute oval opening 

 furnished with a sphincter muscle. It opens immediately below the genital aperture, 

 within an inconspicuous lipped slit, the urino-genital vestibule (Plate VII., figs. 10 

 and 12), placed at the junction of the inner plate of the inner gill with the visceral 

 mass, at a point about mid- way between the ventral border of the latter and the 

 base of the foot. 



Each nephridium consists of a glandular and of a non-glandular portion. By 

 separating the right and left ctenidia and reflecting each, the glandular region 

 (Neph. 1 , Plate V., fig. 4) is seen as a narrow, elongated, coloured strip yellow, or 

 pale brown, or even dark dull red bordering the anterior part of the inner base of 

 each gill. It consists of spongy tissue, occupying the anterior angle formed by the 

 meeting of the inner and outer walls of the organ, and the secretion passes from the 

 cavernous chambers of the glandular region directly into the spacious cavity of the 

 main or non-glandular portion. The spongy renal tissue shows, when magnified, 

 branching tubes and septa formed of irregularly cubical cells much vacuolated and 

 with very distinct nuclei (Plate IX., fig. 15). 



The passage (Np.con., Plate VII., fig. 8) connecting the right and left nephridia 

 lies beneath the auricles. It is a wide tunnel with thin membranous walls, bounded 

 behind by the lower part of the pericardium, while in front its wall lies against the 



