FOEMATION OF SHELLS. 519 



body of Mollusca, differing widely among themselves in construction 

 and habits, but distinguished by a peculiar locomotive apparatus com- 

 mon to the entire class, by means of which they are able to fix them- 

 selves to plane surfaces, and to move from place to place by a slow and 

 gliding motion. The Slug, the Snail, the Limpet, and the Whelk afford 

 familiar examples of their general form and external appearance ; but 

 species of different kinds are so common in every situation, that it 

 would be wasting the time of the reader to dwell at any considerable 

 length upon their ordinary configuration and usual mode of progression. 



(1366.) Many families of Gasteropoda, as for example the NTTDI- 

 BKANCHIATA (fig. 266), 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 cal- 

 culated 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 semifluid 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 dia- 

 meter, 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 texture throughout. 



(1367.) 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. 



(1368.) We choose, as an illustration of the manner in which the 

 external 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 mollusk. The testa- 

 ceous 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. 



(1369.) On making a perpendicular section of one of these Gastero- 

 pods, the entire mechanism by which such shells are constructed and 



