Chap, ix.] STRUCTURE OF SHELLS. 305 



of four years before giving any signs of vitality (Helix 

 desertorum). The shell is sometimes, however, 

 merely chitinous and internal, as in the slug or the 

 squid, or it is, in earlier stages, rudimentary, and in 

 adult life completely lost (Nudibraiichs). In this 

 phylum there is no arrangement comparable to the 

 exuviation which obtains among the Arthropoda. The 

 dependence of these calcareous shells on the nature of 

 their surroundings is admirably spoken to by such 

 facts as the absence of shelled forms from such dis- 

 tricts as the Lizard, or parts of Asia Minor, where 

 there is no lime (Forbes) ; while, on the other hand, 

 the influence of the animal itself, and of other condi- 

 tions, is expressed by the greater density of the shells 

 of the mussel of the m6untam streams of Westmore- 

 land, as compared with the thinner shells of the Isis, 

 which, at Oxford, is richer in salts of lime than the 

 just mentioned more northern waters (Rolleston). 



Shells vary considerably in texture and struc- 

 ture ; many are, as is well known, pearly within ; 

 these nacreous shells owe their characteristic 

 appearance to the alternate deposition of layers of 

 thin membrane and of carbonate of lime, and to the 

 irregular deposition of the thin layers, which, by 

 slightly overlapping one another, diffract the light and 

 so give rise to the iridescence of the internal surface. 

 This explanation of the physics of the appearance of 

 the shell may be applied also to the " pearls," 

 whether formed naturally by the deposition of thin 

 layers around an organic or inorganic nucleus, or 

 artificially by the introduction of foreign material. 

 Where the lustre is duller than in nacreous shells, the 

 hard structures are said to be porcellaiious; in 

 Pinna and others the shell is said to be fibrous, owing 

 to the fact that the separate parts of each layer 

 correspond to one another, and fracture, therefore, 

 results in a number of vertical pieces. The laminated 

 u 16 



