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LETTER XXII. 



ON THE STRUCTURE AND FORMATION OF SHELLS. 



In each of the great divisions of the animal kingdom 

 recognised by zoologists, there is a slightly organised part 

 which, because of its more solid structure, seems intended 

 to protect the vital organs from injury, and to afford points 

 of attachment to the muscles that move the body. In the 

 vertebrated and annulose sub-kingdoms this part or skele- 

 ton consists of many pieces articulated by plain sutures or 

 by moveable joints, and those destined to be the fulcra of 

 the muscles of locomotion are placed in pairs on the sides ; 

 but while the skeleton of the vertebrates is interior and 

 clothed with the muscles, that of the annulose animals is a 

 mere hardening of the skin, and, in many instances, is shed 

 and replaced by a new one, formed under the older, more 

 than once during the insect's or worm's jjrogress to matu- 

 rity. The analogous part in the Mollusca resembles the 

 skin also in being commonly exterior in its position. It 

 is either mucous or coriaceous in texture, or oftener solid 

 and calcareous, consisting of one or two unjoin ted pieces ; 

 but, unlike the skin of the Annulosa, it is not deciduous nor 

 divided into annular segments, and has no closer connexion 

 with the soft parts underneath than what is given through 

 one or two muscles that connect the body with it at one or 

 two points only, yet in such a way that their union during 

 life is indissoluble. The skeleton of the vertebrates is, in 

 fact, an organized portion of the body, penetrated with blood- 

 vessels and absorbents, and, like other organized'parts, ever 

 undergoing change ; but the shell of the Molluscans, although 

 the result of life and organic change, becomes extravascu- 

 lar, so that when once formed it is incapable of being altered 

 by any power inherent in itself. There is this further dif- 

 ference between the skeleton and the shell : — the former is 

 most important in its relations to the function of locomotion, 

 the latter in its use as a covering and shield,* for you will 



* M. Robineau-Desvoidy thinks the shell of the Mollusca the analogue 

 of the vertebral apparatus in higher organisms, hut his speculations are quite 

 unintelligible to me. The reader will find them in the Edinb. Journ. Nat. 

 and Gcogr. Sc. ii. 222, &c. 



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