The animal which inhabits the Pecten shell has the general form of the oyster, differing however from it in a remarkable manner. The edges of the mantle are furnished with multiplied fringes of simple tentacles, between which we find other tentacular appendages a little thicker, each terminating in a sort of small pearl, vividly coloured, to which is attached a nervous thread, which has been taken for an eye. Another difference: the branchiæ, in place of being connected by a striated lamina, as is the case in the oyster, are cut into parallel capillary filaments, forming a free and floating fringe, and the mouth is surrounded by salient many-cleft lips.
Plate XIV.—Pectinidæ.
I. Pecten pallium. (Linn.) II. Pecten purpuratus. (Lamarck.)
III. Pecten foliaceus. IV. Pecten tigris. (Lamarck.)
V. Pecten nodasus. (Linn.) VI. Pecten islandicus. (Chemnitz.)
While the oyster shell is completely fixed to its bed, the Pecten is, on the contrary, perfectly free, and shifts from place to place, moving in the water even with a certain amount of agility; by smartly closing its half-opened valves and forcibly expelling the water, it moves backward by a sort of reaction; this action, repeated many times, compels the animal to move almost in spite of itself, and enables it to avoid danger, or directs its steps towards the spot it wishes to reach. Some naturalists even assert that, when raised to the surface, the Pecten half opens its shell in such a manner that the upper valve serves the purpose of a sail.
Fig. 177. Pecten opercularis (Linnæus).
The Pectens, of which a hundred and seventy-six species are described, are inhabitants of every known sea. Twenty species belong to Europe, among which we may mention P. opercularis, represented in Fig. 177; P. glaber, and P. nivea. Fig. 178 represents the White-mantled Pecten (P. plica, Linn.) of the Indian Ocean, and Fig. 179, the Concentric Pecten (P. Japonica) of the Japan seas.
Fig. 178. Pecten plica (Linnæus).