COTTKEi.L. — Ai/afoiiii/ of Sip]i(inari:i obliciuat; 



585 



The Iddiiev is of a pale-browiusli colour : it consists of two regions, 

 dorsal and ventral, folded one over the other (Plate XXIX, fig. 2 ; dk, vk). 

 The former is attached to the mantle, and near the respiratory orifice, close 

 to the gill, has a small spherical swelling — the renal papilla — on which 

 the renal pore opens (Plate XXIX, fig. 2; rp). The papilla is always more 

 or less concealed by the gill. The ventral region is somewhat leaf-shaped 

 in outline, and is attached in the same way to the floor of the pallial 

 chamber. The whole of the kidney is glandular, and contains no extensive 

 cavity as is seen in the kidney of, e.g., Anodonta or Unio. On the left 

 side the kidney lies close to the pericardium, the right end of which it 

 partially surrounds. 



The gill is composed of a series of laminae separate from one another, and 

 each independently connected to the mantle by one of its lateral edges, the 

 other edge hanging freely into the pallial chamber (Plate XXIX, fig. 2 ; g). 

 As noticed above, this series of gill-laminae forms a semicircle round the 

 posterior end of the pallial chamber. Running round the gill on its posterior 

 side is a large vein — the afferent branchial vein ; and there is another 

 having a corresponding position on the anterior side — the efferent branchial 

 vein. With these vessels the ends of each lamina are connected, the 

 posterior end with the former vessel, and the anterior with the latter. 

 Each lamina tapers at each end to a small tubular structure carrying a 

 branch from one of these vessels on to the lamina. Between these two 



extremes each lamina spreads 

 out and is thrown into longitu- 

 dinal folds, and on the edges or 

 ridges formed by these folds 

 secondary laminae often arise. 

 Dorsally some of these folds may 

 unite with the mantle, especially 

 in the case of those laminae form- 

 ing that part of the gill near the 

 respiratory orifice, where the gill 

 is most strongly developed and the 

 folding is very complicated. 



Each surface of a gill-lamina is 

 covered by a layer of ciliated epi- 

 thelium, consisting of a single layer 

 of cells, which are somewhat 

 cubical, and contain large nearly 

 circular nuclei. These nuclei stain 

 very deeply in borax carmine. 

 Between these two surfaces lies 

 connective tissue co)itaining blood- 

 spaces. There are no skeletal ele- 

 ments in connection with this gill. 

 The blood flows from the posterior vessel into the gill-laminae, and, 

 passing through the blood-spaces in them, is brought close to the water in 

 the pallial chamber, and is thus aerated. 



The gill is ciliated, and the water surrounding it is kept in motion by this 

 means. The blood passes on, and is gradually collected into larger spaces, 

 and finally, reaching the efferent vessel, is carried to the heart. 



This gill seems to be quite different from the typical ctenidium ; it does, 

 however, resemble certam parts of the ctenidia of some opisthobranchs. 



Fig. 



1. — Teaxsverse 



LAMINA ; 



Section of Gill- 

 X 50. 



Anterior renal vein going to the kidney ; 



e, edge of lamina attached to mantle ; 



■ e', free edge of lamina ; rjl, gill-lamina ; 



gV, secondaiy gill-lamina ; m, mantle 



forming roof of pallial chamber. 



