Respiration in Invertebrate Animals. 33 



cated lamina having the same minute structure in every j^art of 

 its extent. In the latter (fig. 3) complex plications {Cj c-b, b) 

 occur which multiply to a considerable degree the area of the 

 active surface. This is so remarkable a character, that if the 

 plicse were a little more prominent, each leaf of the gill of the 

 Periwinkle might be correctly described as a bipinnate structure 

 (fig. 3 B) . The plicae are however mere folds of the smooth sur- 

 face of the lamina, as will b ■ afterwards explained. These parts 

 are so minute, that the malacologist_, using merely the unassisted 

 eye, would pronounce the gill-leaf of Biu^cinum and that of 

 Littorina to be one and the same thing, — to be identically or- 

 ganized. But how essentially unlike ! The evidence furnished 

 by the ultimate anatomy of the branchice would require that the 

 family of the Littorinidse should be placed in juxtaposition with 

 the Tectinibranchiata. 



The pectinibranchiate gill (fig. !«!,«; fig. 2 «, a) may be de- 

 fined then as a series of parallel blood-vessels-bearing leaflets, 

 decreasing in size from the centre of the series to either end, 

 projecting at right angles and vertically depending from the 

 walls of the containing chamber. The long axis of the entire 

 organ is parallel with the line of the rectum and that of the 

 glands peculiar to this cavity (fig. 1 d, b). Though only two 

 main varieties of anatomical structure occur among the branchise 

 of this order, the diversities observable in the size and shape of 

 the laminse in intermediate examples are as numerous not only 

 as the genera but really as the species. In the genus Trochus 

 (figs. 13 & 14) they are more or less similar in all the species. 

 In every species, however, some peculiarity is distinguishable in 

 the contour of the laminae, which suffices to establish specific 

 independence. Those of Trochus magus (fig. 13) are triangular, 

 the dorsal border {a, d) being slightly convex, the free or flexible 

 border {b) being gently concave, while the distal apex is rounded 

 {a). The base (c) of the longest lamina, which occupies always a 

 position in the centre of the gill, measures about y^Qth of an inch 

 in full-grown specimens. 



In Trochus cinerarius (fig. 14), a closely allied species, the 

 distal angle {a) is elongated into an acute apex, the free border 

 (b) is at first convex and then sweeps into a very prolonged base 

 {e, c), giving to the attached border twice as great a length as in 

 that of the former species. Other varieties of figure occur in 

 the gill-leaves of Trochus umbilicatus, and T. tumidus^. The 



* In arriving at a knowledge of the exact figure of the branchial laminae, 

 I have invariably adopted one method of examination. It has consisted in 

 cutting out a few leaves or a small portion of the centre of the gill ; the 

 section being coincident with the plane of the laminae, and at right angles 

 with the long axis of the entire organ. This portion is then carefully 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. xvii. 3 



