VII MOLLUSC A— ^THE NEPHRIDIA 215 



forms, consisting either of glandular protrusions of tlie endothelial wall of the 

 auricles into the pericardial cavity, or of glandular tubes protruding from the 

 anterior corner of the pericardium into the mantle (Keber's organ, red-brown 

 oi^an). The first form is found specially strongly develoj)ed in Mytilus, Lithodomus, 

 and Saxicava, more or less developed in Drcissena, Unto, Anodonta, Vemis, Car- 

 dium, Scrohicularia, Solcu, Pholas, and Teredo, and more or less rudimentary in 

 Pecten, Spond]jlus, Lima, Ostrea. The second form has been observed in Unio, 

 Anodonta, Venus, Cardium, ScroMcularia, Solen, Pholas, Montacuta, and Dreisscnsia. 

 Pericardial glands may also occur singly in other parts of the pericardium, as in 

 Meleagrina (as a projecting ruff in the posterior base of the pericardium), and in 

 Chama on the ventricle, etc. 



The pericardial gland of the Cephalopoda is the so-called appendage of the 

 branchial hearts. This is a structure connected with the branchial heart, and 

 covered with peritoneal endothelium, which projects into the viscero-})ericardial 

 cavity, or, in the Octopoda, into a flask-like widening of the water-canal system 

 (which has been recognised as a division of the ccelom). In Sepia this appendage is 

 conical. A deep furrow on the surface which projects into tlie viscero-pericardial 

 cavity leads into a richly-branched system of canals, the glandular ejuthelium of 

 which is a continuation of the peritoneal epithelium. Blood sinuses from the 

 branchial heart penetrate in between the canals of this system. In other Cephalo- 

 poda, the pericardial gland varies in form and structure ; details of these variations 

 cannot, however, be here given. Nautilus possesses two pairs of pericardial glands ; 

 this fact is connected with its ])ossession of two pairs of gills, with . their two pairs 

 of afl'erent vessels, and on these the two pairs of pericardial glands occupy positions 

 corresponding with those of the branchial hearts. 



XIX. The Nephridia. 

 Kidney, Organ of Bojanus. 



The organs which serve for excretion are homologous in all 

 Mollusca. 



They consist typically of two symmetrical sacs, which, on the one 

 hand, open into the mantle cavity, through the two outer renal 

 apertures, and on the other are connected by two inner apertures 

 (renal funnels, ciliated funnels) with the pericardium or ccelom. The 

 nephridia always lie near the pericardium. Their walls are richly 

 vascularised, indeed a large j^art of the venous blood, in returning 

 from the body, flows through the renal Avails and gives off excretoiy 

 matter before it enters the respiratory organs. The renal walls 

 are traversed exclusively by venous blood. 



The nephridia are paired in all symmetrical Molluscs, and also in 

 those Gastropoda which have paired gills and two auricles (Diotocardia). 



In all other Gastropoda, along with the original right ctenidium 

 (which, in the Prosuhranchia, lies to the left), and the corresponding 

 auricle, only one kidney (the corresponding one) is retained. 



Nautilus, which has four gills and four auricles, has also four 

 kidneys ; only two of these, however, communicate with the viscero- 

 pericardial cavity. 



