36 Dr. T. Williams on the Mechanism of Aquatic 



left of the breathing cavity, while ou the right the rectum, as in 

 Lymneadse, is prolonged in form of a tube beyond the edge of 

 the mantle on the right. In a large specimen of Paludina vivi- 

 para, it is easy to exti-act the animal out of its shell by cracking 

 the latter at diflerent points. The position of the organs con- 

 tained in the branchial vault may be now seen through the 

 mantle. The whole cavity, as in all Pectinibrancbs, is ciliated. 

 On the exterior it is also ciliated to a short distance beyond 

 the edge of the mantle. It lies obliquely in the cavity extending 

 from the posterior left corner to the right anterior end. The 

 gill is constructed in exact conformity with the pectinibranchiate 

 model. The leaves of which it is composed are triangular in 

 shape, the base of the triangle being the free border. They 

 rest on a fixed base, and carry a rigid process of cartilage in 

 the substance of the dorsal margin. The blood-channels and 

 the vibratile cilia exhibit the same disposition as those of Buc- 

 cinum, which will be presently explained at length. The heart 

 is situated at the extreme posterior boundary of the cavity near 

 the point at which the rectum joins the branchia. The ovary, 

 filled with young, is seen on the right side of the rectum. The 

 specimen from which the preceding account has been drawn 

 had been for some time preserved in spirit ; but the author be- 

 lieves that near the dorsal edge of each branchial lamina in 

 Paludina, slight traces of secondary pinnae, or plications, will be 

 discovered, such as those, far more prominently developed, which 

 are now to be ligured and described in the Littorinid?e. If this 

 feature of structure should, on a further examination of fresh 

 examples, be proved to exist, a new point of relationship between 

 the genera Paludina and Littorina will have been established. 

 In describing their respiratory system, it was once intended to 

 place the Littorinidse apart as a separate group, in order that 

 contrast of position might attract towards them immediately the 

 attention of malacologists. The author, however, thinks that, 

 for the present — that is, until, by further search, other examples 

 of the same formation shall have been collected, — it is better to 

 place the Littorinidse here, between the Paludinidse and Turri- 

 tellidse (British MoUusca), rather than dislocate the arrangement 

 of systematists, even at the inconvenience of returning after- 

 wards to the description of the smooth or unplicated variety of 

 branchise. A singular abnormity occurs in the gills of the 

 genus Valvata. It is protruded for a considerable distance 

 beyond the shell, at the left side of the body of the animal. It 

 consists of a long straight axis, from the opposite sides of which 

 filiform pinnae or secondary processes project. These again 

 bear minuter pinnulse, which are the ultimate processes. This 

 gill may be regarded as a transitional variety between the plain. 



