420 



GASTEROPODA. 



Fig. 194. 



shell is entirely effected by the margin of the mantle (fig. 194, 6), 

 which is thick, vascular, and studded with glands appointed to 

 secrete the colouring material that paints the exterior. This 

 thickened fringe of the mantle is firmly glued to the circumference 

 of the opening of the shelly cone : the earthy matter produced by 

 it is added, layer by layer, to the edge of the shell ; and, wherever 

 coloured glands are situated, this earthy secretion is coloured with 

 a corresponding pigment : in this manner is the shell gradually 

 enlarged, and every additional stratum of calcareous deposit is 

 thus painted at the moment 

 of its formation. 



The growth of the shell in 

 thickness is a subsequent 

 process. After the formation 

 of the outer layer (g) by the 

 edge of the mantle, the ge- 

 neral surface of the pallial 

 membrane (a) adds fresh la- 

 minae of pearly matter (/) 



to the whole interior of the testaceous shield, and it is by the 

 accumulation of such colourless depositions that the thickening of 

 the entire fabric is provided for. 



(460.) When the manner in which the limpet constructs its 

 habitation is understood, the formation of a turbinated or spiral shell 

 is explained with the utmost facility. On extracting a snail from 

 its abode, all that portion of its body which was covered by the shell 

 is seen to be 

 invested with 

 a thin mantle 

 (fig. 195, a) 

 precisely ana- 

 logous to that 

 of the limpet : 

 from this pal- 

 lial membrane 

 the nacreous 

 lining of the 

 shell exudes. 

 But around 



the aperture the mantle swells into a thick glandular collar (), cor- 

 respondent in function with the margin of the mantle in Patella, 



Fig. 195. 



