CLASS III-OYSTER, SCALLOP, MUSSEL, ETC.

LAMELLIBRANCHIATA.

THE third and last class of those Mollusks which are headless, comprises those which have their gills in the form of membranous plates, a peculiarity indicated in the scientific name by which the order is distinguished. It includes the oyster, the scallop, the cockle, the mussel, and other well-known bivalves, as such shells consisting of two parts are termed.

The mouth of the oyster is situated near the hinge, beneath a kind of hood formed by the edges of the mantle (Fig. 114).

Fig. 113.-MACTRA.

But the question naturally arises, how is it supplied with food, the animal itself being utterly incapable of any active exertion for that purpose? This is effected by means of rapid currents which are made to sweep over the entire surface of the gills, and hurry towards the mouth the animalcules and particles of nourishing matter which the water contains. By this beautiful arrangement, respiration and nutrition are provided for at the same time.

Shakspeare has said, "honesty dwells like a miser in a poor-house, as your pearl in your foul oyster;" and the connexion of the oyster with the pearl is one of the interesting

circumstances connected with its history. Moore, with his usual felicity, has referred to the Eastern fable of

"That rain from the sky

That turns into pearls as it falls in the sea."

The real facts, as at present known, are scarcely less wonderful. The shell (Fig. 115) is pierced by some worm,

the oyster deposits the "nacre," or mother-of-pearl, on the perforated part; or grains of sand or gravel gain admission into the substance of the mantle, and become encrusted by a similar deposit. This would appear to be, in many instances, the origin of the pearls, so highly prized, and still so eagerly sought for. The Romans were extravagantly fond of these ornaments, which they ranked next to the diamond, and are said to have given almost incredible prices for them.

OYSTER, SCALLOP, MUSSEL.

The common Mussel of our coasts lives moored to its "bed" by the silken cable which it con

structs for this purpose.

The silk which it produces is, however, much surpassed by that of the Pinna, a shell already mentioned (page 60).The silk of the Pinna has been woven into some articles of dress, that in early times were so highly

prized as to be worn only by emperors and kings. At Taranto, in Italy, it is still mixed. with about one-third of real silk, and made into gloves, caps, stockings, &c.

Among the Mollusks of the present class, are those which possess the art of boring into hard substances, and living in the excavations thus formed. The one most noted for its ravages is the Teredo (Fig. 116), which, by its power of perforating wood, does great injury to ships, piers, and all submarine wooden buildings: owing to the general use of metal sheathing, it is now nearly extinct on the British shores.

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Fig. 116.-TEREDO.

It is occasionally the pleasing duty of the naturalist to direct attention to some of the many examples, where there springs from "partial evil, universal good;" and perhaps the Teredo, notwithstanding the evidence of its destructive powers, might, if the whole truth were known, be ranked among the number of our benefactors. Mr. R. Ball has remarked, "that, but for the maligned Teredo, the sea would be so covered with floating logs of timber, as to be to some extent unnavigable; that the rivers of warm latitudes would be choked up by the accumulated driftwood at their mouths, and that their fertile banks would, in many cases, be converted into morasses."

On one occasion, during a stroll along the beach, the waves flung upon the shore a piece of the painted woodwork of some unfortunate vessel. On examining it, I found that it was pierced throughout by the Teredo, and that the animals were still living in the galleries they had excavated, and which were lined with shelly matter throughout all their windings. While each animal had used with effect the curious auger-shaped instrument by which the perforations were made, no one had interfered with the progress of his fellows; and almost in every instance, when the borings approached too close, their direction had been changed, and contact or interference thus avoided. That piece of drift timber, the sport of winds and waves, contained within itself a little world of animated existence!

Having now noticed the three classes of the Mollusca which are without a distinct head (acephalous), we proceed to those which have a distinct head (encephalous); of them there are likewise three classes.

CLASS I PTEROPODA. THE little Mollusks belonging to this order are furnished with two membranous expansions, like fins or wings (Fig. 117), and hence the compound term, which signifies "wing-footed," points out the obvious distinguishing characteristic of the class.

Fig. 117.-HYALEA.

There are several genera; but the species best known (Clio borealis) is about an inch in length, and so abundant in the Arctic seas as at times to colour the surface for leagues, and to form an important supply of food to the great whale. The head has six of the wing-like appendages, which appear of a reddish tint from the number of distinct red spots scattered over their surface, and amounting on each to about 300. Every one of these specks, when examined under a microscope, is found to consist of about twenty suckers, each of them mounted on a footstalk, so as to be applied to their prey; so that the total number of suckers on the head of one of these animals is about 360,000, an apparatus for the capture of food without any parallel in the animal creation.