vii MOLLUSC A THE NEPHEIDIA 219 



(b) Monotocardia. The Monotocardia have only one nephridium functioning as 

 an excretory organ, viz. the left of the Diotocardia. This takes the form of a sac 

 lying immediately below the mantle cavity on the right side of the pericardium, 

 directly under the integument. It is generally found to the left of the hind- 

 gut ; less frequently (Cassidari", Tritoniidce) the kidney is traversed by the rectum, 

 or the latter runs forward below it. The slit-like pallial aperture of the kidney, 

 however, is always found to the left of the hind-gut, quite at the base of the mantle 

 cavity. This position of the kidney, and especially of its outer apertures, had 

 already led to the assumption that the Monotocardian nephridium corresponds with 

 the left kidney of the Diotocardia, before this fact was established. The assump- 

 tion was all the more plausible because of the occurrence of a gland called the 

 anal kidney in a few Monotocardia (e.g. Dolium) ; this gland opens to the right 

 near the anus, and might represent the right kidney of the Diotocardia. 



The kidney is always connected by means of a canal (the reno-pericardial canal) 

 with the pericardium. 



Lamellse or trabeculse, covered with the glandular epithelium of the kidney, 

 project inward from the lateral walls of the renal sac. -These are especially 

 strongly developed in fresh -water Prosobranchia (excepting Valvata), traverse 

 the whole kidney, and impart to it a spongy structure. The venous blood always 

 flows through the whole of the glandular part of the kidney, either in special 

 vessels or in lacuna, before passing on to the gills ; but an open communication 

 with the renal cavity is never found. 



In the Tcenioglossa Proboscidifera the kidney forms two lobes similar in struc- 

 ture. In Natica and Cyprcca the lobes begin to differ, and among the Stenoglossa 

 this difference becomes more and more marked in a way which need not here be 

 described. 



In Paludina and Valvata the kidney no longer opens into the posterior base of 

 the mantle cavity, but is continued as a urinary duct (ureter), which runs forward 

 in the mantle and opens at its edge. 



The above-mentioned theory that the single kidney of the Monotocardia corre- 

 sponds with the left kidney of the Diotocardia has recently been ably opposed, 

 another theory being put forward in its place. Attention is specially drawn to the 

 fact that in the Diotocardia the left kidney is always the smaller, that in Patella it 

 is shifted to the right side of the pericardium, and that in Haliotis, Turbo, and 

 Trochus (as papillar sac) it is not excretory in function. In Haliotis, Turbo, 

 Trochus, and Po.tdla the lacunar system developed in the wall of the left kidney is 

 in direct communication with the auricles. 



In most Monotocardia there is a differentiated part of the kidney, viz. that 

 which is called the nephridial gland. This consists of two principal parts : (1) 

 canals, covered with ciliated epithelial cells and opening into the kidney. These are 

 merely protrusions of the renal wall, which project into the organ ; their epithelium 

 is a continuation of the renal epithelium. (2) Between these canals, the organ is filled 

 with cells of connective tissue and muscles, and contains blood lacunae, one of these 

 being specially large and communicating with the auricle. This latter portion of 

 the organ perhaps plays the part of a blood-forming gland. 



This nephridial gland may perhaps be the persistent excretory portion of the lost 

 nephridium, i.e. the right of the Diotocardia,. The duct of this lost nephridium is 

 now known to persist as genital duct. As we saw above, all Diotocardia discharge 

 the genital products through the right nephridium. 



2. Pulmonata (Fig. 182). The Pulmonata have only one kidney, which lies 

 in the mantle at the base of the pallial cavity, between the rectum and the peri- 

 cardium. The renal sac is of the so-called parenchymatous type, the excretory 

 epithelium of its wall projecting into the cavity in the form of numerous 



