The nervous system is here more simple than in the animals of the preceding classes. There are no ganglia scattered through the body ; all the nervous filaments taking their rise either from the medullary mass above the entrance to the stomach, or from the one beneath. Although the body does not appear liberally supplied with nerves, yet the sense of touch is exquisite, parti cularly in those filaments or tcntacula with which the margin of the cloak, or its apertures, are provided. No organs of sight or hearing have been observed.
The food of the Conchifera appears to be obtained exclusively from the substances floating or mixed with the water. These are brought within the sphere of the mouth by the movements of the cloak. The mouth it self is destitute of proboscis, jaws, tongue, or even lips. Around its margin are four tentacula, which, in struc ture, bear a considerable resemblance to the branchia. The mouth opens immediately into the stomach, without the intervention of any thing which can be called a gul let. This organ is usually full of cells; the bottom of each pierced with a biliary duct. The liver surrounds the stomach, and empties its bile by numerous openings. A singular crystalline process, cylindrical, cartilaginous, and transparent, projects into the cavity of the stomach, whose use has not been determined. There is some times an enlargement, in the form of a second stomach. The intestine, in some species, makes several convolu tions; in others, particularly those which are fixed, it is remarkably short. The anus is placed in the extremity of the body, opposite to the mouth, and either opens into the cavity of the cloak, or into one of its syphons.
The only organ of reproduction hitherto observed in this class, is the ovarium. This occupies the sides of the body, and penetrates the membranes of the cloak. The eggs pass into the gills, where they are hatched, and, bursting the integuments, make their escape. In the ovarium has been observed, at certain seasons, a milky fluid, regarded as sperm. There appears to be no reason to infer the existence of any thing like sexual union. Lamarck is supposed to consider fecundation as effected by means of an impregnating fluid mixed with the water, which must therefore have been ejected from male organs. This supposition, however, is neither sup ported by facts nor analogy.
The most important of the peculiar secretions of the animals of this class, is the Pearl. This substance,
equally prized by the savage and the citizen, is com posed, like shells, of carbonate of lime, united with a small portion of animal matter. Pearls appear to be exclusively the production of the bivalve testacea. Among these, all the shells having a mother-of-pearl in side, produce them occasionally. But there are a few species which yield them in the greatest plenty, and of the finest colour. The most remarkable of these is the Avicula margaritifera. This shell, which was placed by Linnaeus among the mussels, is very widely distri buted in the Indian Seas ; and it is from it and another species of the same genus, termed Avicula hirundo, found in the European seas, that the pearls of com merce are procured. The Pinna, so famous for furnish ing a byssus or kind of thread, with which garments can be manufactured, likewise produces pearls of considera ble size. They have seldom the silvery whiteness of the pearls from the Avicula, being usually tinged with brown. But the shell which in Britain products the finest pearls, is the Unio margaritifera, which was placed by Linnaeus in the genus Iklya. It is found in all our alpine rivers. The Conway and the In in.England, the rivers of Tyrone and Donegal in Ireland, and the Tay and the Ythan in Scotland, have long been famous for the production of pearls. These concretions are found between the membranes of the cloak of the animal, as in the Avicula, or adhering to the inside of the shell, as in the Unio. In the former case, they seem to be a mor bid secretion of testaceous matter ; in the latter, the matter appears to be accumulated against the internal opening of some hole, with which the shell has been pierced by some of its foes. Linnaeus, from the consi deration of this circumstance, endeavoured, by piercing the shell, to excite the animal to secrete pearl ; but his attempts, though they procured him a place among the Swedish nobility, and a pecuniary reward, were finally abandoned ; the process being found too tedious and un certain to be of any public utility. The largest pearl of which we have any notice, is one which came from Pa nama, and was presented to Philip II. of Spain, in 1579. It was of the size of a pigeon's egg. Sir Robert Sibbald mentions his having seen pearls from the rivers of Scot land as large as a bean.