XII 



BACKBONELESS ANIMALS 



259 



Lima arc active compared with the oyster, and they have 

 thinner shells ; the sea-snails which creep between tide- 

 marks or on the floor of the sea are heavily weighted, 

 while the sea-butterflies (Pteropods) have light shells, 

 and most cuttlefish have- none at all. 



The shell is very distinctive. In most of the embryo 

 molluscs which have been studied there is a little pit or 

 " shell-gland " in which a shell begins to be formed, but 

 the shell of the adult is in all cases made by a single or 

 double fold of skin known as the " mantle." In some 

 cases where the shell seems to be absent, e.g. in some 



FIG. 78. THE ROMAN SNAIL (Helix pomatia), ONE OF THE TERRESTRIAL 

 AIR-BREATHING [PULMONATE] GASTEROPODS. 



It creeps along on its flat muscular ventral surface, the so-called foot 

 (F). The head bears retractile long horns (LH) with an eye (E) at the tip, 

 and retractile short horns (SH). The breathing opening is on the right- 

 hand side near the mouth of the shell and the food-canal ends at the 

 same place. 



slugs, a vestige is still to be found beneath the skin, 

 while in other cases (e.g., the Octopus) there is no trace 

 left. There are two or three primitive forms (Chcetoderma, 

 Neomenia] where an incipient shell is represented only by 

 a few spicules or plates of lime. 



The shell is a non-living product of the skin ; it 

 consists for the most part of carbonate of lime along 

 with a complex organic substance called conchin or con- 

 chiolin ; it shows typically three layers, of which the 



