EXCEETOEY OEGANS OF MOLLUSCA. 



377 



manent kidney — has beeu made out in the Palmonata. In adult 

 animals tlie organ is, as a rule, unilatei'al. This double rudiment 

 points to its being the same as the paired organ of the Lamelli- 

 branchiata. The recent discovery of a paired excretory organ in 

 Haliotis, Fissurella, and Patella, in which forms the left organ is 

 more or less rudimentary, seems to be decisive on the matter. The 

 degeneration of one of the two organs appears to be correlated with 

 the degeneration of other paired organs, as, for example, the gills. 

 So far as exact observations show, it opens by one pore into the 

 pericardial sinus, and by the other to the exterior. In the majority 

 of the Gasti-opoda, uric acid has been detected in the organ. This 

 is especially true of the Pulmonata, in which the kidney, which is 

 j)laced between the heart and pulmonary veins, can be easily recog- 

 nised on account of its whitish or yellowish colour. It is lamellar or 

 spongy in structure, and the lamella3, or bands which make it up, are 

 covered by large secreting cells, in which firm concretions of various 

 forms can be made out. The long efferent duct, which commences 

 some way back, opens into the pulmonary cavity, which in Helix 

 appears to be a widened terminal portion of this duct. 



In the Prosobranchiata the kidney lies between the gills and the 

 heart ; it has the same position in some of the Opisthobranchiata. 

 As a rule an efferent duct passes forwards and accompanies the hind- 

 gut, below which, and often not far behind the anus, we find its 

 orifice. 



In many of the Opisthobranchiata it appears to have lost its 

 excretory function (as in Polycera), or the excretion is fluid. In this 

 case the kidney (as in Phyllirhoe, Actteon, etc.) has the form of a 

 long transparent tube, which extends some way back behind the 

 heart along the middle line of the body, and is placed close to the 



Fig. 200. Diagram of Doris, to show the excretory organ B. o Moutli. I Lips. 

 B Buccal mass, ce ffisophagus. v Stomach, i Intestine, a Anns. at Anricle. 



V Ventricle (after A. Hancock). 



back. It opens into the pericardial sinus by a ciliated orifice, and 

 by a contractile one on to the surface of the body. 



In many of the Opisthobranchiata a greatly-developed ctecal-sac 

 is given off by this organ, which gives off secondary diverticula 

 (Fig. 200, B), and so gradually forms a ramified tube. Structures of 

 this kind, which resemble a ramified gland, have been observed in 

 Doris, Dendronotus, Scyltea, etc. A canal (B') is continued from the 



