GASTEROPODA. 419 



DIBRANCHIATA (Jig. 185), are absolutely deprived of any shelly 

 defence, the investment of their bodies being entirely soft and 

 contractile. In others, as the slug (Limax), a thin calcareous 

 plate is imbedded in the substance of their muscular covering. 

 This little shell is contained in a cavity within the mantle, and is 

 quite loose and unattached to the walls of the cell wherein it is 

 lodged. The mode of its formation and growth is exceedingly 

 simple, and from its very simplicity is well calculated to illustrate 

 the formation of shells of more complex character. The floor of 

 the cavity containing the calcareous plate is vascular, and secretes 

 cretaceous particles mixed up with a viscid animal secretion. The 

 material thus furnished in a semi-fluid state is applied like a layer 

 of varnish to the lower surface of the shell already formed by the 

 same process ; and the added layer, soon hardening, increases the 

 thickness of the original plate, while at the same time, as a necessary 

 consequence of the progressive extension of the secreting membrane, 

 which enlarges with the growth of the slug, each successive lamina 

 of shell is larger than that which preceded it. Thus the extension 

 of the shell in diameter, as well as its increase in thickness, is easily 

 explained. In these internal shells, however, there is no colouring 

 matter ; so that they are uniformly white, and present the same tex- 

 ture throughout. 



(459.) As external shells are generally painted upon their outer 

 surface with colours of different kinds variously disposed, in such 

 the process of growth is somewhat more complicated, and in every 

 essential particular resembles that already described, whereby the 

 shells of the CONCHIFERA are extended in size and thickness. 



We choose, as an illustration of the manner in which the exter- 

 nal shells of univalves are manufactured, one of the least complex 

 forms, as being best adapted to elucidate this part of our subject. 

 The Patella, or common limpet, is covered with a simple conical 

 shell that extends over the whole of the dorsal surface of the mol- 

 lusk. The testaceous shield that thus protects these animals is 

 generally variegated externally with sundry markings of diverse 

 colours, while within it is lined with a smooth and white nacre. 



On making a perpendicular section of one of these Gasteropods, 

 the entire mechanism by which such shells are constructed and 

 painted is at once rendered intelligible. The whole of the back of 

 the animal covered by the shell is invested with a membranous 

 mantle, like that of a conchiferous mollusk ; but different parts of 

 this mantle are appointed to different offices. The extension of the 



