781 



SHELL. 



SHELL. 



762 



what are regarded zoologically as distinct families; but where a 

 marked diversity does exist, I believe that it will always be indicative 

 of the affinities of the animal. Thus the conformity in structure 

 between all the shells of one natural family is usually BO close, that 

 any strongly marked difference in a particular genus would make me 

 hesitate in admitting it into the group." Dr. Carpenter further adds 

 that although the characters derived from the structure of the shell 

 are not likely to serve for the distinction of species from each other 

 or even genera, for the distinction of some families they furnish the 

 best natural characters at present possessed by the naturalist. 



In examining the structure of shells, very thin sections of the 

 shell must be obtained, and on these being placed under the micro- 

 scope, it will be found that they are translucent, and permit of the 

 passage of the rays of light. This is the case with all shells, and is 

 indicative of their crystalline character. This shows that the dis- 

 tinction of shells into crystalline and granular or concretionary, as 

 proposed by Dr. J. E. Gray, is not founded on a knowledge of the 

 microscopic structure of the shell, although for practical purposes it 

 serves to distinguish between certain forms of shell. When it is 

 wished to examine the animal matter of shells independent of their 

 inorganic constituents, the shell should be immersed in dilute hydro- 

 chloric acid, which leaves the animal matter and takes away the 

 calcareous. This process Dr. Carpenter calls ' decalcifying.' 



The animal matter of the shell exhibits itself more particularly in 

 two forms, the Cellular and the Membranous. In most cases amongst 

 the MMiaca the cellular structure assumes a prismatic form. " If," 

 says JJr. Carpenter, " a small portion be broken away from the thin 

 margin of the shell of any species of Pinna and it be placed without 

 any preparation under a low magnifying power, it presents on each 

 of its surfaces, when viewed by transmitted light, very much the 

 aspect of a honeycomb ; whilst at the broken edge it exhibits an 

 appearance which is evidently fibrous to the eye, but when examined 

 under the microscope with reflected light, resembles that of an 

 assemblage of basaltic columns. The shell is thus seen to be com- 

 posed of a vast multitude of prisms, having for the most part a 

 tolerably regular hexagonal shape and nearly uniform size. These 

 are arranged perpendicularly to the surface of each lamina, so that 

 its thickness is formed by their length, and its two surfaces by their 

 extremities." The substance forming these prisms is generally trans- 

 parent, but here and there is found a dark spot. This arises from 

 the fact that the prisms are found to be composed of a series of cells 

 laid one on the top of the other. In the majority of the cells the 

 calcareous matter is deposited in a crystalline form, so as to give it a 

 translucent character, but occasionally this object is not effected, and 

 the cell is filled with air, and presents the dark appearance known 

 to be produced under the microscope by transparent vessels contain- 

 ing air. This prismatic cellular substance is formed upon the external 

 surface of the mantle or skin of the animal, and may be regarded as 

 homologous with the epithelial cells, which constitute the epidermis 

 of the higher animals, and is called by Dr. Carpenter a ' calcified 

 epithelium.' That these cells are deposited by successive layers appears 

 from the fact that, when tho decalcified layers are examined they 

 exhibit a series of transverse lined not unlike the transverse stria; of 

 muscular fibre. 



In the shells which do not present the prismatic cellular tissue 

 there is seen a layer of membranous shell-substance. In this layer 

 no cells can for the most part be discovered. In no shell is there an 

 entire absence of this organic basis, and Dr. Carpenter suggests that 

 at one time this membrane was a constituent part of the mantle of 

 the Mollusc. It appears to be homologous with the ' basement mem- 

 brane' found in the mucous membranes and the skin. This membranous 

 substance presents a variety of appearances ; sometimes it is single, as 

 if the shelly matter had been uniformly diffused over a plane surface. 

 It is more frequently uneven, sometimes amounting simply to a 

 corrugation or wrinkling, at other times the wrinkles are so strongly 

 marked that they give the appearance of a series of fusiform cells, as 

 is well seen in the shell of the. common Patella. 



What is called the Nacre in shells, and which is more especially 

 observed in their interior, and which is distinguished by the iridescence 

 produced by the action of light, is found to consist of layers or folds 

 of this membranous shell-substance. When examined under the 

 microscope, this substance exhibits a series of layers, and it has been 

 supposed that these are alternately layers of membranous and calca- 

 reous matter; but Dr. Carpenter has shown that when these layers 

 are decalcified they exhibit the iridescence, but that when the corru- 

 gations are removed by stretching, the membrane no longer exhibits 

 its beautiful hues. 



The membranous shell-structure 'IB often traversed by tubes which 

 vary from the j^noth to the n^ co th of an inch in diameter. Their 

 direction and distribution is exceedingly various, but in general they 

 form a kind of network, which spreads itself out in each layer. At 

 other times these tubes are quite straight. They have been regarded 

 aa perforations ; but Mr. ISowerbank and Dr. Carpenter have both 

 seen in them indications of a cellular structure, and regard them as 

 originating in the coalescence of a series of cells. This structure is 

 not often observed in the nacreous substance. 



A structure of the shell-substance occurs in Rudistes which Dr. 

 Carpenter calls ' cancellated structure.' It resembles the prismatio 



cellular substance, but the cells are hollow instead of being filled up. 

 Dr. Carpenter says he has not met with this structure in a decided 

 form amongst recent shells. 



The Pearls found in the interior of the shells of many species of 

 Mollutca seem to result from the deposit of the nacreous substance 

 around some nucleus mostly of foreign origin. They are probably 

 always formed between the mantle and the shell, although sometimes 

 found in the muscles or viscera of the animal. The Avicula margari- 

 tifera is called the Pearl Oyster, on account of the frequency with 

 which pearls are found in its interior. [AvicuLA.] Pearls are not 

 unfrequeutly found in the common oyster, and most fishmongers 

 possess specimens which they have obtained from oysters they have 

 opened. Many of the fresh-water Conchifera of our own rivers contain 

 them. The Unio margaritiferus yields them. It is said that the 

 reputation of British pearls excited the cupidity of Julius Ciesar, and 

 that he presented a buckler covered with them to Venus Genetrix, 

 which was suspended in her temple. Pliny speaks of British pearls 

 disparagingly. Those obtained from the Unio are generally very good; 

 but many are procured from the Mytilm cdulis, and these are not so 

 valuable. (Forbes and Hanley, 'British Mollusca.') 



With regard to the growth of shells, although it is true, generally 

 speaking, that shells cover the embryo of the testaceous mollusc iu 

 the egg, as observed by Swammerdam, Pfeiffer, and others, such is not 

 its condition in all cases. In Argonauta, for instance, the shell is not 

 coeval with the first formation of the animal. [OcTOPODA.] 



Dr. J. E. Gray, who states that the shells of Mollusca appear to bo 

 coeval with the first formation of the animal, observes that the 

 Cephalopodous Mollusca form no exception ; their bone, composed of 

 two or three calcareous plates, being found fully developed in the 

 cuttle-fish some time before the young animal is hatched. These 

 observations, he adds, are directly at variance with the theory main- 

 tained by Sir Everard Home (' Phil. Trans.,' 1817), namely, that 

 the shell is formed after the animal has quitted the egg ; and, as 

 regards the cuttle-fish, they are opposed to the remark made by Baron 

 Cuvier, that the young cuttle-fish, when first hatched, has only a 

 cartilaginous plate like the Loligo. 



The shell, when first observed on the embryo (even of the animals 

 of spiral shells), forms, Dr. Gray observes, a short blunt more or less 

 curved sub-cylindrical cone, covering the hinder part of its body. As 

 the organisation of the embryo becomes developed, and the hinder part 

 of the body extended, the shell, he remarks, increases iu size, till the 

 body and shell together occupy nearly the whole of the egg. " While 

 in the egg," says Dr. Gray in continuation, " the embryo shells are 

 generally of a pale horn colour, and destitute of markings ; when 

 therefore they remain attached to the apex of the spire of adult 

 shells, they may be easily distinguished by their appearance from the 

 part formed after their exclusion ; and as in such cases they offer some 

 characters of importance, it has been proposed to designate them by 

 the name of the nucleus of the shell. The effect of the atmosphere 

 on the shell is almost instantaneous. In some young Helices, and iu ii 

 species of Valuta, iu my collection, the very first line of calcareous 

 matter deposited after their exclusion from the egg is marked nearly 

 as the adult shells of the species." 



The same author remarks that the nucleus which forms the original 

 apex in all shells, and frequently remains attached to them during all 

 the periods of their growth, is largest in those shells the animals of 

 which are viviparous, and is consequently very distinct in Voluttv, 

 Palud'mce, and Cydades. In the oviparous species, he observes, it 

 agrees in size with the egg of the animal ; thus Achatina octona, 

 which has an egg nearly equal in dimensions to the mouth of the 

 shell, &n&Bu,linu ovalus and 11. bicarinatus, which lay large eggs, have 

 large nuclei, the magnitude of the nucleus in general rendering the 

 top of the spire blunt. Some, on the contrary, as Stylina generally, 

 and Pupa purpurea, have, he adds, a very long slender acute turret^d 

 nucleus. The form and size of the eggs of these molluscs do not 

 appear to be known. 



The nucleus consists of two coats or parts : the external coat or 

 layer, Epidermis and Periostracum of authors, is of a somewhat horny 

 or membranous character ; the inner layer constitutes the true shell. 

 This epidermis is thinnest in such shells as are enveloped in the 

 mantles of their auimals. The gradual growth of the shell so consti- 

 tuted is effected by the secretion from the mantle. The preparation 

 No. 93 A, in the Museum of the College of Surgeons in London, 

 exhibits a Cyprcea Tigris with the soft parts. One of the lobes of the 

 mantle, the secreting organ of tho shell, is protruded. There are 

 'lands at intervals in the mantle of those shells that are ornamented 

 with coloured patterns in tho form of necklaces or stripes, which pro- 

 duce those patterns in many cases with all the correctness of a design. 

 But though the mantle is the ordinary secreting organ, the vitreous 

 external coat giving the highly glazed appearance to which the olives owe 

 so much of their beauty is secreted by the foot, and not by the mantle. 



The plates which form the rhombic crystalline shells are, Dr. Gray 

 observes, deposited in succession, each gradually increasing in thick- 

 ness as the shell enlarges. As the animal waxes in size, the lip 

 gradually shelves, becoming thinner from the inner to the outer edge, 

 she innermost part being formed of three layers, the next of two, and 

 the outer and thinnest part (that which is first formed) of only a 

 single layer. At the approach of the periodical stoppage of the 



