260 



THE STUDY OF ANIMAL LIFE CHAP. 



LH 



outermost is somewhat soft 

 and without lime, while the 

 i n n e r most shines with 

 mother-of-pearl iridesce nee. 

 Between the two is the pris- 

 matic layer with the lime 

 laid down in prisms. Un- 

 like the shell of Arthropods 

 that of molluscs does not 

 require to be moulted as 

 the animal grows, for it is 

 always being added to at 

 the margin. 



Most molluscs are marine, 

 on the shore, in the open 



FIG. 79. A DARK VARIETY OF THE 

 GREY SLUG (Limax maximus), 



TO BE CONTRASTED WITH A 



SNAIL (Helix). 



The shell in this form is repre- 

 sented by a thin plate concealed 

 beneath the mantle (M), which en- 

 closes a pulmonary or respiratory 

 chamber. In the Black Slug, Arion 

 ater, there is no vestige of a shell. 

 In Helix the anterior margin or 

 " collar " of the mantle secretes the 

 shell. The short horns (SH) and 

 the long horns (LH), with the eyes 

 at the tip, are clearly shown. 



sea, in the great depths ; 

 there are also many fresh- 

 water forms, e.g. the mus- 

 sels Anodon and Unio, and 

 the snails Limncea, Planor- 

 bis, and Paludina ; the 

 terrestrial snails and slugs 

 are legion. Among molluscs 

 of the shore the naked 

 Nudibranchs are often in 

 colour and form protec- 

 tively adapted to their sur- 

 roundings ; those of the 

 open sea (Heteropods, 

 Pteropods, and m any 



