134 INTRODUCTION TO ZOOLOGY CHAP. 



This is strikingly illustrated by the well-known case of 

 the specimen of the Desert Snail (Helix desertorum), which 

 was sent to the British Museum in 1846, and fixed to a tablet 

 there. Four years afterwards, when examining this mounted 

 snail, Mr. Baird saw signs of the recent formation of an 

 epiphragm; he removed the snail and put it in warm 

 water, when it revived and came out of its shell ! The next 

 day it ate a meal of cabbage, and before long was quite 

 normally active again, and mending its somewhat broken 

 shell. 1 ' 



Slugs. 



Amongst the univalve Mollusca, slugs are peculiar on 

 account of the rudimentary condition of the shell, which is 

 represented in most genera merely by a small calcareous 

 plate, or even by a few spicules only, hidden below the 

 shield -like mantle. This mantle lies on the top of the 

 long body ; its margin is fused with the body all round, 

 except where the respiratory aperture is left on the right 

 side, leading into the lung- cavity lying below the mantle 

 (Fig. 87). The theory that the shell has, in the ancestors of 

 these forms, been more fully developed, is upheld, not only 

 by the vestiges of a shell that still remain, but by the fact 

 that in an early stage of the development of each individual 

 a distinct spiral shell is present. 2 



In all slugs the wrinkling of the surface of the flesh is 

 very marked, and also there is always one very distinct groove 

 running parallel to the margin of the body, marking off the 

 sole of the foot. 



In the Common Black Slug (Arion ater), and in some 

 others also, this border is marked by vertical lines, alternately 

 black and dusky, forming what is known as the foot fringe 

 (Fig. 87). 



The tentacles are similar in form and function to those of 

 common garden snails. 



Reproduction in slugs is generally very rapid. 



One P air of tlie comraon black s l u g (4rion ater) 

 was kept under observation, and after pairing, it 

 was found that one of the slugs laid 396 eggs in five separate 



1 Ann. Mag. Nat. Hist., (2) vi., 1850. 

 2 Taylor, vol. i. p. 201. 



