GROWTIL 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 against 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 
