436 ABSORPTION OF THE SHELL. 



which are removed before the mouth of the shell is con- 

 tinued. 



In some shells, however, which have only short processes, 

 as in the variety of the Pyrula bucephala with two rows of 

 spines, the front rows are not absorbed, the inner lip being 

 deposited of such a thickness as to cover them. A simi- 

 lar circumstance may be observed in a monstrous variety of 

 S trombus pugilis, with two rows of spines, of which there 

 is a specimen in the British Museum. 



In the young state of the Fissurellse, the hole by which 

 the faeces pass out of the shell is placed a little in front of 

 its recurved and spiral apex : in this state it has been formed 

 into a genus under the names of Rimula and Puncturella. 

 But as the animal grows the hole enlarges in size backwards, 

 and the true apex being absorbed, the hole appears in the 

 adult shell to be placed on the tip, and in some species even 

 to extend behind it. 



The animals of many species absorb parts of their shell 

 at regular periods : thus the Tritons, which at each of the 

 periodical interruptions of their growth form a thickened 

 edge to their lips, when they again commence enlarging 

 their shells, generally absorb this thickening both as regards 

 that part which had been deposited on the pillar, and that 

 which formed the ribs and teeth of the outer lip ; for on 

 examining the cavity of any of these shells it will be found 

 quite smooth and free from interruption. Such an absorp- 

 tion does not, however, take place in some of the larger 

 Cassides, and in the genus Persona, in which the thickening 

 of the former lips remains after the shell has enlarged in 

 size, and forms prominent bands on the parietes of its cavity. 

 But a similar periodical deposition and absorption of the 

 thickening of the outer lip takes place in many of the land 

 shells, as the Helices and Bulimi ; in most of which there is 

 formed, at every interruption of their growth, an internal 

 rib, just within the edge of the mouth, which is removed 

 when the animal again begins to increase its shell. This is 

 particularly visible in the genus Scarabus, where the inter- 

 ruptions are regularly periodical, each period of growth 

 occupying half a whorl, as in the Ranellae. 



Mollusca not only have the power of absorbing their own 

 shells, but they also possess the faculty of forming cavities 

 in those of other animals. When a specimen of Pileopsis 

 attaches itself to the surface of a shell, it generally leaves 

 in the place of its attachment a depression of its own size, 

 and furnished with a horse-shoe-shaped ridge : such cavities 

 are sometimes formed even in other specimens of their own 



