GEOWTH OF SHELLS. 289 



are placed the glands through which colouring matter is added 

 to the lime of which the shell consists, and here also the whole 

 of the outer coat of the shell is formed by constant annual 

 additions to the lip. The after-growth of the shell proceeds, 



Chinese Wentle-trap. — (Scalaria pretiosa.> 



layer over layer, from the general surface of the mantle, so that 

 the calcareous robe constantly increases in thickness with the age 

 of the animal. 



However different the form of a shell may be, its use is in- 

 variably the same, affording the soft-bodied animal a shield or 

 retreat against external injuries. In this respect it is not 

 uninteresting to remark that those species which inhabit the 

 littoral zone, and are most exposed to the violence of the 

 waves, have a stronger shell than those which live in greater 

 depths, and that the fresh- water molluscs have generally a 

 much more delicate and fragile coat than those which live ■ in 

 the ocean. The greater the necessity of protection the better 

 has Nature provided for the want. Thus most of the gastero- 

 pods, besides possessing a stone-hard dwelling, are also furnished 

 at the extremity of the foot with an operculum, or calcareous 

 lid, which fits exactly upon the opening of their house, and 

 closes it like a fortress against the outer world. But no 

 animal exists that is safe agaiDst every attack, for the large 

 birds sometimes carry the ponderous sea-snails, whose entrance 

 they cannot force with their beaks, high up into the air, 

 and let them fall upon the rocks, where they are dashed to 

 pieces. 



The ordinary mode of locomotion of the testaceous sea-snails- 



