520 



GASTEROPODA. 



Fig. 260. 



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 shell is 

 entirely effected by the margin 

 of the mantle (fig. 260,6), 

 which is thick, vascular, and 

 studded with glands appointed 

 to secrete the colouring mate- 

 rial that paints the exterior. 

 This thickened fringe of the 

 mantle is firmly glued to the 

 circumference of the opening of Section of Patella. 



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. 



(1370.) 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 general surface of the pallial membrane (a) adds fresh laminae of 



Fig. 261. 



Helix pomatia, removed from its shell. 



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. 



(1371.) "When the manner in which the Limpet constructs its habi- 

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

 explained with the utmost facility. On extracting a Snail from its 



