40 Dr. T. "Williams on the Mechanism of Aquatic 



incident of structural type as a criterion of relationship between 

 the several genera of the family of the Littorinidge. He has 

 not examined the branchiae of Lacuna, Assiminia, Jeffreysia and 

 Skenea ; but those of the genus Rissoa discover a marked ten- 

 dency towards this peculiarity of formation. 



Several families must now be passed over as hitherto unex- 

 amined. The branchiae of the Muricid.ne may probably, however, 

 be considered as typically representative of those groups which 

 intervene between tliem and the Littorinidse. 



The whole of this extensive family is said by systematic authors 

 to be characterized by the possession of two gills ; one described 

 as doubly pectinated, and the other as singly pectinated. As 

 formerly stated, Dr. Sharpey has adopted this definition in his 

 article " Cilia," in the ' Cyclopaedia of Anatomy and Physiology/ 

 The branchial chamber in this family (fig. 2) is constructed on 

 the same principle as that of other Pectinibranchs. It is a 

 capacious vault, open in front from one side to the other by a 

 fissure. On the left side the edge of the mantle is prolonged 

 into an extended recurved siphon. The glands (c, b) of the 

 cavity, as will be explained afterwards, are highly developed ; 

 one of them so nuicli so as to have led to the idea that it was a 

 second branchia. The true gill {a, a), which is a single organ, 

 stands between this supposed supplementary gill and the large 

 mucous gland (c), to the left of which is observed the rectum (e). 

 In the Muricidpe, the third gland (fig. 1 g), called by some 

 authors the renal gland, is not visible within the boundary of this 

 cavity. The whole of the interior of the chamber is actively 

 ciliated ; the exterior is not so. The epithelium here is smooth. 

 Little peculiarity exists in the branchial system of this family 

 (fig. 4). The organ is large (fig. 2 a, a); it has the shape of 

 two cones laid base to base. The broadest laminse are therefore 

 in the middle, the smallest at either end. It is so situated as 

 to receive directly the column of water as it enters by the siphon 

 (/). The course of this water, as indicated by the arrows (fig. 1), 

 is from left to right; it thus passes first, and in the most pure 

 state, over the branchise, then over the mucous gland, and lastly 

 over the rectum. The planes of the branchial laminae (fig. 4) 

 are coincident in direction with the main water-current in pass- 

 ing from the left to the right side of the branchial cavity. 

 Nothing is so easy as to determine the figure of the gill-leaves 

 in any of the larger genera of this family. Buccinum is a famihar 

 example. 



The gill of Buccinum nndatwn is composed of many hundreds 

 of leaves. These leaves, towards the centre of the organ, are 

 regularly triangular in figure (fig. 4 a, a). At the extremities, 

 each lamina loses more rapidly in vertical depth than in length, so 

 that at last they become scarcely visible creases of the pallial mem- 



