THE SLUG AND ITS ALLIES 



163 



and greenhouses. Particularly in Europe, Limax agrestis 

 has often devastated fields of young shoots ; this species 

 is especially fond of bulbous plants. 



In the apparent absence of a shell the slug seems to be 

 an aberrant gastropod. Other land gastropods the 

 snails have an evident 

 shell. In Limax the shell 

 is reduced to a thin, horny 

 plate, embedded in the 

 mantle. Between the con- 

 dition seen in the snail 

 and that in Limax there 

 are intermediate condi- 

 tions, in which the large 

 shell is partly covered by 

 the mantle, and others 

 in which the shell has 

 become reduced in size. 



In allies of Limax in FlG - 149 - ~ Illustrating the gradual 



covering of the shell (sh) in certain 

 a geilUS Called Anon Opisthobranchs by the epipodia (ep) and 



the shell is reduced to a raantle ; **> cephalic disc. Drawn to 



various scales. A, Hammea; B, Sca- 

 few Calcareous grains. phander; C, Aplustrum; I), Aphysia; 



An exactly similar series ^ Philine ' From Cooke ' " Mollus ^-" 

 in the degeneration of the shell is found in certain Opistho- 

 branchs, illustrated in Fig. 149. The beginnings of this 

 process of covering the shell are seen in many gastropods 

 in which the mantle edges may protrude beyond the 

 lips of the aperture, and are folded back over the outer 

 surface of the shell. A more developed condition is seen 

 in species like Aplysia, in which the mantle is permanently 

 reflected. 1 In Limax the reflected edges of the mantle 



i Fig. 149a. 



