240 Natural History of Molluscous Animals : — 



lies in the canal or emargination, and that is formed by a 

 prolongation and duplicative of the cloak. 



The gills of the Cephalopoda are placed, one on each 

 side, within the sac, to which they are attached by a thick 

 broad fleshy riband. Each gill is composed of two series of 

 branchial leaves connected on one side to the fleshy riband 

 just mentioned by short pedicles, and on the other uniting 

 with the leaves of the opposite row ; and, as the leaves of the 

 one series are not opposed to, but alternate with, those of the 

 other, each of them unites two leaves of its antagonist series; 

 and they are in this manner all joined together at the margin 

 farthest from the fleshy riband, and along which the branchial 

 vein runs. Each leaf is itself garnished with little cross 

 leaflets, and these are similarly divided (Cuvier, Mem. sur les 

 Mollusques, vol. i. p. 20.); the structure, on the whole, 

 reminding one of the mechanism of the gills in fishes. 



The branchiae of the bivalved Mollusca are always placed 

 between the body and its cloak, the folds of which, being, in 

 many of them, altogether separate in front, admit the circum- 

 fluent water very freely ; but when these folds are soldered 

 together at the edges, as they often are, the water is imbibed 

 through a siphonal tube formed by an elongation of the 

 cloak, and extruded at the posterior end of the shell ; and 

 the effete fluid is expelled again through the common 

 excrementitious tube, which is altogether like the other, and 

 occupies the same position. In the Branchiopoda, a small 

 order of this class, the branchiae are arranged in a pectinated 

 form on the inner surface of each lobe of the cloak ; but the 

 other tribes have free gills in the form of large semilunar 

 leaves that embrace the sides of the body. There is a pair 

 of these, often of unequal size, on each side, and each leaf is 

 joined to that which corresponds to it on the opposite side at 

 the dorsal margin ; but in front they are usually separate : 

 they are broad and lamelliform, are finely and regularly 

 striated across, and sometimes appear punctulated in the 

 intervals of the striae. * Each leaf, according to Blainville, 

 is itself formed of two layers which leave between them 

 a free space, divided by numerous triangular partitions into a 

 great number of vertical cells open to the dorsal margin. 

 These layers are constituted by two series of parallel vertical 

 vessels united by others which cross them ; one of the series 



* These striae appear to be formed of a small number of tubes, bound 

 together. They may be compared to the nervures of the wings of insects ; 

 for they seem partly intended to keep the branchiae even, and prevent them 

 being rumpled. They are parallel to one another, and connected by cross 

 tubes, that divide the interspaces into small regular squares. 



