The mantle of the animal is free, with double margins, the inner of which forms a finely fringed curtain all round, and on this curtain are a number of black eyes surrounded by very fine tentacles. The gills are in the form of very thin crescents, and the foot is shaped like a finger.
Although the majority of scallops are inhabitants of tropical seas, several species are to be found off our coasts, where they range from depths of about four to forty fathoms, and the empty shells, often in the most perfect condition, are frequently found on the beach.
The Common Scallop (P. maximus) is largely used as food, and is therefore a common object in the fishmonger’s shop. Its colour is very variable, and the shell has equal ears and about twenty radiating ribs. The Quin (P. opercularis) is also an important article of diet in some parts.
Perhaps the prettiest of the British species is the Variable Scallop (P. varius), so called on account of the very variable colour of the shell, the ground tint of which may be almost anything between a very pale yellow and a dark reddish brown, and this is irregularly patched with some lighter colour. The chief distinguishing features of the species are the spiny projections of the numerous ribs, most prominent near the margin of the valves, and the presence of a permanent byssus, which, in other species, occurs only in the young. Three of the species named above are shown on [Plate V.]
We may also mention the Tiger Scallop (P. tigrinus), the radiating ribs of which are sometimes slightly formed, and which has only one ear in each valve; and P. pusio, in which the adult shell is often greatly altered in form.
It may be noted, in conclusion, that all the species of this genus have the power of swimming rapidly by flapping their valves—a mode of locomotion very common among the bivalves especially during an early stage of their existence.
Before passing on to the univalve molluscs, we must refer briefly to a group of animals that are enclosed in bivalve shells, and which were once included with the Mollusca, but are now made to form quite a distinct group by themselves. We refer to the Brachiopods, at one time very abundant, as proved by the immense number of fossil shells embedded in various stratified rocks, but now represented by only a few living species.
The shells of these animals are commonly known as Lamp Shells, on account of their resemblance to an antique lamp; and although at first sight they bear a general likeness to certain bivalve shells of lamellibranchs, a close examination will show that not only the shell, but also the animal residing within it, are both of a nature very different from that of the molluscs with which they were at one time supposed to be closely related.
