THE COMMON SXAIL. 21 



The opening at the bottom, from which the animal iesiies, 

 is the last portion finished, and this is called the aperture, 

 a Latin word adopted into the English dictionaiy. Some of 

 these Helices are wide and flat, even hollow and cup-like, 

 with the whorls rising above the pillar — these are called 

 discoid shells; others which are long and narrow, with pro- 

 jecting spires, are termed tiirhinated shells; the former being 

 more or less flat or disk-like, the latter twisted, whirling, hke 

 a spinning-top, from the Latin turho — a whirling, a turning 

 round. 



TURBINATKD. 



If we take a Common Snail, and plunge it into boiling 

 water, which will instantly kill it, so that it can be re- 

 moved from the shell, we shall find the whole of that part 

 of the body which was lodged in the upper whorls, or spiral 

 part of the shell, is covered with a thin membrane or skin; 

 this is called the mantle, and that portion of it which cor- 

 responds with what we may consider as the back of the 

 mollusk, and which is considerably thickened, is termed the 

 collar; here are situated the glands, which secrete the colouring 

 and other matter of which the shell is mainly composed; 

 although the substance called nacre, or mother-of-pearl, is 

 secreted in the thinner part of the mantle; it is however from 

 the collar that the growth or increase of the shell proceeds. 

 It is in accordance with certain variations in the shape and 

 disposition of this mantle and collar, that shells assume such 

 very diff'erent shapes. Sometimes the whorls or spiral ridges, are 

 projected or thrown far out, and this produces the tnrhinated 

 shell. Sometimes they scarcely rise above each other, but 

 rather spread towards the sides, and then we have the discoid 

 shape. Generally speaking, the whorls of a shell take a direc- 

 tion from left to right, but occasionally an opposite one; 



