114 



CONCHOLOGY. 



to defend certain delicate organs most subjected to 

 injury. Some writers have considered them analogous 

 to the bones of other animals, although its formation 

 and growth arc widely different, since it serves as a 

 base or support to the muscles which are attached to 

 its internal surface. 



Reaumur, to whom we are indebted for numberless 

 interesting and accurate observations of the pheno- 

 mena of nature, thus describes the increase of shell 

 which takes place after the development of the animal 

 and its progress of growth : " The enlargement of 

 shells after this period is owing to successive additions 

 of calcareous and animal matter, independent of any 

 organised structure. When a testaceous animal is 

 about to enlarge its shell, the common snail for 

 instance, and its body has become too large to be 

 covered securely, it projects a portion of its body from 

 the opening ; it then attaches itself to a wall or other 

 solid substance, and the naked part is soon covered 

 with the fluids which are excreted from its surface ; 

 the pellicle, or covering, which they produce, when 

 the fluid dries, is, at first, thin and elastic, but gradu- 

 ally assumes more consistence, and becomes at last 

 similar to the whole part of the shell. If, in this stage 

 of the process, a bit of the shell is broken off and 

 removed without injuring the body of the animal, the 

 skin of the snail is soon covered with a fluid which 

 gradually thickens and becomes solid. In about 

 twenty-four hours after the operation a fine crust may 

 be observed, which constitutes the first and external 

 layer for repairing the breach that was made ; at the 

 end of some days this layer becomes thicker, and in 

 ten or twelve days, under favourable circumstances, 

 the new piece of shell has acquired the same thick- 

 ness as that which was removed, but it never unites 

 in the same way, being only as it were a plug. If, 

 after the broken piece has been removed, particularly 

 if the fracture is made near the edge of the opening, 

 the animal is not supplied with a sufficient quantity 

 of nourishment, its bulk is soon diminished, and 

 now finding what is left of the shell equal to com- 

 pletely cover its lessened body, no exertion takes 

 place for the production of a new portion. It is 

 obvious, snails, &c., cannot enlarge in volume but by 

 the augmentation of the whorls, and that each previ- 

 ous turn of the spire remains the same in length ; 

 these make a great difference in the size of the shell, 

 by the last and additional one, each being calculated 

 to contain nearly double that of the preceding one ; 

 but in many shells, both marine and river, the last 

 whorls of the spire, compared with the preceding ones, 

 greatly exceed this proportion. In some, the external 

 opening is twelve times greater, or from eight to 

 twelve : this depends entirely on the increase of the 

 animal's body, and the duration of that increase. 

 Those testaceous animals which have only a few turns 

 in the spire of the shell are of this description ; to the 

 former belong such as have a greater number of turns ; 

 the growth of shells, being proportioned to that of the 

 inhabitant, proceeds almost imperceptibly. In most 

 shells this, however, is marked on the convex side 

 with lines, more or less fine, giving the shell a fibrous 

 appearance ; these are called striae, which may be 

 traced through the whole of the shell, in bivalves and 

 in the longitudinal direction of those which have a 

 spiral form ; they cannot receive any addition without 

 that progress being discovered on a minute examina- 

 tion. These inequalities must become visible, for the 

 smallest addition of testaceous matter which is made, 



must be attached to the old part of the shell, and con- 

 sequently, must be more elevated than the former ; 

 whatever be its thickness, the increase, or rather the 

 interruption of the growth is thus distinctly marked; 

 and they have some degree of analogy with the 

 different shoots of trees, checked by the cold of winter 

 or advanced by the heat of summejr, as is remarked 

 in such shells which live on the land, or inhabit rivers 

 in temperate regions ; in these the shell is not 

 enlarged during those seasons ; it is otherwise, 

 however, with regard to its thickness, for that is con- 

 tinually going on, from the constant exudation of 

 small quantities of fluid from the animal's body ; this 

 addition is generally marked also by a fresh coloured 

 stripe or band, darker than the present one." 



That which appears certain is, that the growth of 

 shells is extremely slow, and that their inhabitants are 

 able to exist a very long period without food, parti- 

 cularly such as can screen themselves entirely from 

 exterior circumstances, as is the case with the Helix. 

 This is the more remarkable as the animal has not 

 a stock of fat to exist upon as in some hybernating 

 animals ; that substance never existing in molluscs. 

 Numberless instances of this could be adduced, and 

 among others proving the tenacity of life in snails, 

 the writer has received some of their species closely 

 packed with marine shells from the Mauritius, which 

 the day after their being unpacked were discovered 

 wandering about, apparently uninjured by their con- 

 finement. Some lived ten days, but a want of proper 

 food, or more probably an uncongenial climate, then 

 killed them. Another instance is recorded in the 

 Philosophical Transactions, of snails having resumed 

 their wonted habits, after having been imprisoned in 

 a cabinet fifteen years ; this is not, however, vouched 

 for by the writer of this article. 



These remarks are so easily corroborated by every 

 day's observation, that they need not be enlarged 

 upon, and those who are disposed to examine some of 

 nature's interesting operations, will do well to begin 

 with such as are not attended with philosophic cruelty ; 

 and as the formation of the snail's shell illustrates the 

 growth of all similarly constructed habitations, a more 

 attainable object cannot well be pointed out. 



Having, as we think, explained the mode of increase 

 adopted by the animals of a large proportion of the 

 various genera of shells, we will now shortly describe 

 a difference which exists in another very extensive 

 division, called, by some naturalists, porcelain shells, 

 on account of their smooth surfaces and high polish, 

 resembling china-ware, in these there are two sets of 

 colours, disposed in a parallel direction to each other. 

 The external range of these colours is owing to a 

 peculiarity of structure in the animals which inhabit 

 them, different from that of other testaceous animals, 

 and to an operation which does not take place in 

 other shells. In these shells the colouring matter 

 seems to be always deposited in two different ways, 

 and at two distinct periods. In the first process, 

 when the body of the shell is formed, the colouring 

 matter is excreted from the glands in the body of the 

 animal, and it is arranged according to the disposition 

 of those glands. At this period the shell is only of 

 a very moderate thickness, much less than it after- 

 wards acquires when completely formed. On -the 

 external surface of the shell thus formed, another layer 

 is deposited, which is more compact than the former, 

 in some places thicker than in others, and usually 

 variegated with different colours ; the external shell 



