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M O L L U S C A. 



Mollusca. 



IV. CLASS. CONCHIFERA. 



The molluscous animals which are destitute of a head, 

 were separated, at an early period, by Cuvier, into a 

 distinct order, to which he gave the name of Acephala. 

 Subsequent observations having pointed out the char- 

 acter of being acephalous, as common to animals which 

 differ widely from one another in the arrangement 

 and disposition of their other organs, it became neces- 

 sary to introduce into the system a more definite me- 

 thod of arrangement. Accordingly, Lamarck, in his 

 " Hisloire Nalurclle ties Animaux sans Vertebras," v. 

 p. 41 1, instituted the class CONCHIFERA, which includes 

 nearly all the inhabitants of the bivalve testacea, or 

 those whose gills are in the form of leaves, four in 

 number, and disposed in pairs externally, on each side 

 the abdomen, and within the cloak. As the bivalve shells 

 were long known to naturalists under the denomina- 

 tion Conchae, the propriety of the term to designate the 

 class, employed by Lamarck, is sufficiently obvious. 



The common integuments of the Conchifera, consist 

 of the cloak and shell. The cloak forms two leaves, 

 one on each side the body of the animal, united behind. 

 This cloak is, in some families, open in front, while, in 

 others, it is united ; perforated, however, by holes, or 

 tubular elongations termed syphons, for admitting wa. 

 ter and food. Corresponding with the two sides of the 

 cloak, are the two valves of the shell. These valves 

 are likewise united behind with an elastic ligament, 

 which aids the animal in opening and shutting them. 

 The shells are attached to the animal by the anterior 

 margin of the cloak, which adheres to the margin of 

 the shell, and by the adductor muscle. This muscle, 

 which passes across the body from one valve to another, 

 brings the valves, by its contractions, into contact, at 

 their free edges, at the same time that the ligament is 

 compressed or stretched, according as it is internal or 

 external. When the muscle is relaxed, the ligament 

 exerts its power, and opens the valves ; and along with 

 the valves, the cloak to which they are attached. The 

 adductor muscle is in some families divided, and the 

 two portions separated from each other. 



Locomotion is performed in some to a limited extent, 

 by suddenly opening and shutting the valves In ge- 

 neral, however, those species which shift their place, 

 are furnished with a muscular projection from the body, 

 capable of changing its shape, and attaching itself to 

 foreign bodies, termed a foot. The base of the foot is 

 usually attached by two or more tendinous filaments to 

 the shell. The kystus issues from a muscular body, like- 

 wise connected by filaments with the shell, and is fixed 

 to other bodies. The foot is supposed to be the organ 

 which spins this thread; but its mode of formation is 

 involved in obscurity. While some are permanently 

 fixed, and others are capable of moving from one pi-ice 

 to another, there are a few which prefer a residence in 

 d'fferent substances in which they have excavated a 

 habitation. These last are termed Borers. It was sup- 

 posed by many that the animal secreted a liquor with 

 which it dissolved the bodies into which it penetrated; 

 but the sagacious Reaumur soon ascertained that the 

 boring was performed by means of a rotatory movement 

 of the larger valves. M. Fleurieu Bellevus states, that the 

 calcareous stone, in which the Rupellaria lithophaga is 

 found, is often discoloured in the immediate neighbour- 

 hood of its recess. This may arise from the secretions 

 of the animal, or even from the stagnant sea water in 

 the hole, and not from the action of the phosphoric 

 acid, or any other solvent supposed to be employed by 



the animal. These solvents would act equally on the Moliusca. 

 shell as on the calcareous rock. But the borers are not T""" 1 ' 

 confined to calcareous rocks : they also lodge in slate- 

 clay, and other argillaceous strata. This is very often the 

 case with the Pholades. But this character can never be 

 extensively employed in the distribution of gentra, as 

 the same species which, at one time, may be found im- 

 bedded in stone, will be observed, at another, seated 

 among the roots of sea weed, or buried among gravel. 



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 tentacula 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 

 itself is destitute of proboscis, jaws, tongue, or even 

 lips. Around its margin are four teiitacula, which, in 

 structure, bear a considerable resemblance to the bran- 

 chia. The mouth opens immediately into the stomach, 

 without the intervention of any tiling which can be 

 called a gullet. 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 nurner- 

 ous openings. A singular crystalline process, cylindri- 

 cal, cartilaginous, and transparent, projects into the ca- 

 vity of the stomach, whose use has not been dete'rmined. 

 There is sometimes an enlargement, in the form of a 

 second stomach. The intestine, in some species, makes 

 several convolutions ; 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 sex- 

 ual union. Lamarck is disposed to consider fecunda- 

 tion as effected by mans of an impregnating fluid 

 mixed with the water, which must therefore have 

 been ejected from male organs. This supposition how- 

 ever is neither supported by facts nor analogy. 



The most important of the peculiar secretions of the p ear i. 

 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. A- 

 mong 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 

 Avicnta martraritifera. 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- 



