Groioth and Structure of Shells. 



It will be seen by the foregoing arrangement that the Mollusca occupy 

 the second grand division of the Animal Kingdom, and, ranging next in order 

 to the Eishes, are the most higldy organized of the invertebrate series. They 

 are termed Mollusks, from the Latin word mollis, soft, on account of their 

 fleshy unjointed nature, without bone, or, except in the Cephalopod, any 

 internal cartilaginous skeleton ; and are furnished with organs of motion, 

 vision, muscular contraction &c. Their chief energies are, however, de- 

 voted to the formation of a shelly covering presenting great variety of form, 

 colour, and sculpture. Some mollusks, as the Aplysiana, have only a ru- 

 dimentary shell, whilst others, as the Eolida, are entirely naked. 



The shell is formed by the deposite of a slimy juice which exudes from 

 the glands of a filmy cloak-like organ called the mantle, and thickening in 

 successive layers, becomes hardened and moulded on the body, with the ad- 

 dition of such ornamental structure, as the various filamentous modifications 

 of the organ promote. 



This mucous exudation consists, not, as at one time supposed, of calca- 

 careous particles held together by a sort of animal glue, but of a mem- 

 branous tissue, consolidated by an admixture of carbonate of Hme. All 

 shells are therefore composed of two distinct elements, — animal matter and 

 calcareous matter. The animal matter constitutes a membranous basis 

 which is either cellular or laminary ; in some cases it is formed in cells of 

 membranous walls, mostly hexagonal, into which the calcareous matter is 

 infused; in others it is deposited in membranous lamina?, more or less 

 wrinkled like morocco leather, with the calcareous matter in alternate layers, 

 the lime in both cases imparting solidity to what would otherwise be a 

 flexible stratum. These elements vary in their relative proportions accord- 

 ing to circumstances ; — in those shells which are of a fibrous laminary struc- 

 ture such as Puma, and the whole of the margaritaceous group, there is a large 

 proportion of the membranous constituent ; whilst in the solid porccllanous 

 shells, such as the Olives and Cowries, the calcareous matter preponderates, 

 the parts in immediate contact with the acetose juices of the mantle becoming 

 vitrified, as it were, to a higldy polished state of enamel. Great importance has 

 been attached to the elementary structure of shells by the recent microsco- 

 pical investigations of Dr. Carpenter*; his experiments have been most 

 successfully conducted by grinding down a thin layer of shell, and placing it 

 in dilute acid, the result being that the calcareous matter or Hme decomposes, 

 leaving a residuum of membranous tissue. On the other hand, he des- 

 cribes an instance of a mass in which the animal basis, a cellular tissue, had 



* Report of the fourteenth Meeting of the British Assoeialion p. 1. pi. 1 to 20. 



