182 THE MECHANICAL FUNCTIONS. . 



piring by means of pulmonary organs. In the young shell 

 of this tribe, the partitions which separate the cavities of the 

 whorls are incomplete, and twine parallel to each other; but 

 they wholly disappear as the animal approaches to maturity. 

 In other cases, the animal is found to remove exterior por- 

 tions of shell formerly deposited, when they lie in the way 

 of its farther growth, and when the mouth of the spire is 

 advancing over the irregular surface of the preceding whorls. 

 Thus we often find that the ridges, ribs, or processes which 

 had been deposited on the surface of the shells of the Tri- 

 ton, Miirex, &c. are removed to make way for the succeed- 

 ing turn of the spire. In other cases, however, no such 

 power of destroying portions of shell previously deposited 

 seems to exist; and each successive whorl is moulded upon 

 the one which it covers. 



It may also be observed, that some mollusca have the 

 means of excavating the shells of other animals on which 

 they may choose to fix, for the purpose of forming a conve- 

 nient lodgement for themselves. The Fileopsis (or fool's 

 cap) has this faculty in a remarkable degree; and it is also 

 met with occasionally in SlphonarHce and Patellar. The 

 common Patella^ or limpet of our own coasts, often, indeed, 

 forms for itself, by some unknown process, a deep cavity 

 out of a calcareous rock. 



When the animal which inhabits a spiral shell retires 

 within it, the only part of its body that is exposed to injury 

 is that which is situated at the mouth of the shell. With 

 a view to its protection, it constructs, in many instances, a 

 separate plate of shell, adapted to the aperture, and denomi- 

 nated an Operculum. This piece is constructed by a pro- 

 cess similar to that by which the rest of the shell is formed; 

 that is, by the deposition of successive layers on the inter- 

 nal surface, sometimes in an annular, and sometimes in a spi- 

 ral form. If an operculum were to be constructed of a consi- 

 derable size, and were connected to the shell itself by a re- 

 gular hinge, it would be entitled to be considered as a dis- 

 tinct valve. Here, therefore, we perceive, as was remarked 

 by Adanson, a connecting link between the univalve and 



