218 



COMPARATIVE ANATOMY 



CHAP. 



nephridia opening into the mantle cavity to the right and left of the anus. The 

 left nephridium is, however, much reduced, while the right, which is strongly 

 developed, sends its lobes everywhere into the spaces* between the lobes of the liver, 

 the intestine, and the genital organs. There are no reno-pericardial openings. The 

 genital gland does not open direct into the mantle cavity, but through the right 

 kidney. 



In Haliotis, Turbo, and Trochus, both nephridia are present. The left nephri- 

 dium has, however, almost entirely lost its excretory function, but is still connected 

 both with the pericardium and the mantle cavity. It is called the papillar sac, its 

 walls projecting into its cavity in the form of numerous large papilla?. The blood 

 lacuna? which penetrate into the papilla? communicate direct with the auricles, and 

 are thus supplied with arterial blood. In these lacuna? of the papilla? a crystalloid 

 substance (albumen ?) is deposited. It has been thought that these papillar sacs 

 serve as reservoirs of nutritive material (in the form of the crystalloids just men- 

 tioned), and when needed yield it up to the blood. 



The right nephridium is exclusively excretory in function. It is divided into 

 two lobes, one behind the other, which communicate by means of a wide aperture ; 

 the anterior lobe lies under the floor of the mantle cavity, bulging it upward. A 

 spongy network, covered with excretory epithelium, rises from part of its wall into 

 the cavity of the nephridial sac. The meshes of the network are penetrated by a 

 system of vessels with walls of their own. Nearly all the venous blood, before 



reaching tin- gills, passes through the 

 vascular system thus developed on the walls 

 of the kidneys. The right nephridium is 

 in no way connected with the pericardium. 

 The Neritidse have only one nephridium 

 to the right of the heart, which opens 

 through a slit in the base of the mantle 

 cavity. The renal sac is traversed by trabe- 

 culae, many of which reach from one wall to 

 the other, forming a spongy structure. The 

 trabecula? are covered by a glandular epithe- 

 lium on the surfaces turned to the spaces of 

 the sac. 



Patella (Fig. 181) still has two nephridia, 

 both functioning as excretory organs. The 

 apertures lie at the two sides of the anus. 

 FIG. 181. Diagram of the two nephridia The right kidney is, however, much larger 



of Patella (after Lankester). ksa Anterior 

 and upper lobe of the large right kidney KSl ; 



T} both He to the 

 J 



of 



tori, lower subviseeral ; iJp, posterior lobe of the pericardium, but there are no reno-peri- 

 the same ; /, subanal tract of the large right cardial apertures. The internal structure 

 kidney ; g, anal papilla with the portion of the o f the right kidney is spongy, but the left 



forms a simple cavity, into which folds 

 project from the walls. A lacunar system 

 without special walls traverses the trabecular 



the existence of the reno-pericardial aperture network of the right kidney, but is com- 

 pletely cut off from its cavity ; the venous 

 blood from the body passes through this 

 The lacunar system of the left kidney communicates 



rectum which runs to it ; h, papilla with the 

 aperture of the left kidney (which is not 

 drawn) ; /, the same of the right kidney ; I, 

 pericardium, indicated by a dotted outline ; 



figured near /, is now denied. 



system before entering the gills, 

 directly with the auricle. 



In Haliotis and Patella also the genital products pass, as in Fissurella and the 

 Diotocardia generally, out of the genital gland into the right kidney, and are ejected 

 through the right renal aperture. 



