MORPHOLOGY OF MELIBE 555 



(3) The Venous System. 



The venous system seems to consist of a numher of very 

 thin-walled sinuses so that the blood easily exudes through 

 them, bathing the surrounding organs. The efferent branchial 

 veins collect the blood from the sinuses of the papillae, and 

 perhaps also from the larger sinuses of the body-cavity. The 

 so-called pericardium lies closely below the mid-dorsum, and in 

 front of the intestine and the ureter, and above the anterior 

 branches of the kidney. I have not at this time determined 

 the exact nature of the pericardium and its relation to the 

 blood, whether it is a completely closed chamber or not ; 

 whether it is invested with its own peritoneal membrane, or 

 whether it is fenestrated, allowing blood to enter it from the 

 surrounding sinuses. It is, in fact, usually thought that 

 the pericardial space in the molluscs contains blood, and is in 

 free communication with veins ; but Lankester (1893) has 

 succeeded in showing by observations on the red-blooded 

 S o 1 e n 1 e g u m e n , and by more recent careful investigation on 

 Anodonta cygnea, 'Patella vulgata, and Helix 

 a s p e r s a , that the pericardium has no communication with 

 the vascular system and does not contain blood. 



8. The Organs of Excretion. 

 (1) The Kidney. 



According to Pelseneer (Lankester, 1896 : 111) the kidney 

 is a compact mass, as a rule, without external projections, 

 but it is divided into lobes in Stenoglossain general, and 

 in some Taenioglossa , viz. Paludina and Cypraea. 

 In a fairly large number of nudibranchs (Doridomorpha, 

 Janus, &c.) the kidney is divided into ramifications which 

 extend between the visceral organs of the greater part of the 

 body. Shipley and MacBride (1915) say the kidney is a 

 vesicle, into the cavity of which numerous folds project covered 

 by the peculiar cells which have the power of extracting waste 

 product from the blood, which flows in spaces in the kidney wall. 

 The kidney in Mollusc a varies a good deal in structure, but 



