110 Mr. A. Hancock on the Animal of Chamosti*ea albida. 



though I would not positively assert that this is its natural con- 

 dition, for the animal had been removed from the shell before 

 it came into my possession, and these parts are so delicate that 

 very little violence is sufficient to rupture them. I could not, 

 however, perceive the least appearance of laceration, and there is 

 therefore little doubt that the above description is correct. 



It is difficult to say whether the gill-plate represents the 

 outer or the inner plate of other Lamellibranchs. In Mytilus, 

 which has two perfect gill-plates, there is also an external rudiT 

 mentary gill formed, as in this instance, of a single lamina. It 

 would seem probable, therefore, that the breathing organ of 

 Chamostrea represents the outer plate together with the rudi- 

 mentary gill of that genus. 



The branchia of Chamostrea being reduced to little more than 

 a single gill-plate on each side of the body, some complication of 

 the apparatus might be expected as a compensation for the 

 deficiency ; and such is found to be the fact. If the laminse 

 composing the plate are divided, their inner surface is found to 

 be formed of a reticulation of blood-vessels running at right 

 angles to each other ; those (Fl. IV. fig. 4 a) extending across 

 the lamina, predominating considerably in size, give to the sur- 

 face a corded appearance in a transverse direction, the cords 

 being separated by a series of apertures (c). On turning the 

 lamina over, the outer surface is ascertained to be covered with 

 numerous delicate transverse plaits or folds (fig. 3 a), so crowded 

 together as to conceal the true surface. When a transverse 

 section of the lamina is made, these plicse (fig. 5 a, a) are seen to 

 coincide with the spaces between the large transverse vessels seen 

 on the under surface of the lamina, and are consequently placed 

 over the series of apertures {d, d) between these vessels. The 

 plicae themselves are formed of double walls, enclosing a wedge- 

 shaped space with its base towards the lamina. These walls are 

 made up entirely of a very minute network of vessels (fig. 6 a, h) 

 which cross each other at right angles, the meshes being open, 

 and elongated in the direction of the plicae. This structure is 

 perfectly similar to that of the gill-laminae of Mya, Pholas, &c. ; 

 and the nutritive particles will be strained from the surrounding 

 element by the surface of the plicae, just in the same way as the 

 gill-lamina itself strains the water in other Lamellibranchs ; that 

 is to say, the water by ciliary action will be made to pass through 

 the apertures or meshes, leaving behind all sedimentary matters, 

 and, escaping into the wedge-shaped spaces between the walls of 

 the plicae, will find its way at once, by the series of apertures 

 between the large, transverse, cord-like vessels of the gill-laminae, 

 into the interbranchial spaces or tubes ; and as these latter open 

 freely into the great dorsal or posterior channel which comma- 



