AN ORGANIZED BODY. 387 



The shell of the Mollusca, under every modification of 

 form, consists of carbonate of lime and animal matter in va- 

 riable proportions ; but "the two are not uniformly diffused 

 through the shell as some have supposed, but are separate 

 elements, each having its own place." Since the publication 

 of the experiments of Reaumur, in 1709, the general opinion 

 with conchologists has been that the lime and mucilaginous 

 fluid was excreted solely from a series of glands seated in 

 the collar or margin of the mantle of the animal, and depo- 

 sited, by the same part, on the rim of the shell, by which 

 means it was enlarged in the particular form ordained to 

 characterise its species. There was first exuded a line or 

 layer of mucous fluid which, hardening by exposure, be- 

 came the skin or periostracum of the future shell; under- 

 neath this there was then laid a layer of calcareous matter 

 which made itself solid by aggregation or a process of crys- 

 tallisation, and within this another layer and another, until 

 the requisite degree of thickness was attained. The whole 

 of the material was derived from the glands in the collar, 

 and by the collar only was that material added, particle by 

 particle, to the edges of the shell, and spread along its inte- 

 rior, the latter effected as the animal successively withdrew 

 far within its cell and again protruded itself. Thus the 

 shell was an excretion, and its solid state being the result of 

 a mechanical condensation, it was consequently an inorganic 

 structure. 



There were, however, some opponents to this theory, 

 Herissant, the contemporary of Reaumur, maintained that 

 shell has an organic structure, and that it grows by intersti- 

 tial deposit in the manner of bone ; and Adanson appears to 

 have held the same opinion. If, he says, we consider the 

 shells to be the bone of Mollusks, we must look upon the 

 membrane which covers the greater part of them to be their 

 periosteum ; and, in fact, this membrane performs the same 

 function, since it contributes both to the shell's preservation 

 and growth.* So, also, Bradley tells us that " Shells have 

 a kind of vegative growth, and have also vessels of communi- 

 cation with the animal they contain, so that it seems as if 

 the juices of the one were necessary for the support of the 

 other." ( Bradley, probably, borrowed his belief from 

 Lister ;J but the celebrated Poli held the same, maintain- 



* Hist. Nat. du Senegal, xliv. 



t Phil. Account, 51. 



J " Per hanc insuper fasciam ipsa testa nutrimentum sutim assumat, 

 necesse est ; quod inter earn ipsumque animal nulla alia communicatio 

 detur." Exer. Anut. de Cochleis, 18. 



c c 2 



