Shell-Bearers 



we read its story correctly there is a wealth of tragedy in 

 this half shell. It tells of a life-and-death struggle on the 

 part of the oyster to get the better of an insidious enemy 

 in the shape of a small orange-coloured sponge. This 

 little sponge has the objectionable habit of settling itself 

 upon an oyster shell and, having done so, of boring in- 

 numerable tunnels in its surface. The bivalve, in a vain 

 attempt to repair the damage, spends laborious days in 

 forming layer after layer of shell to keep pace with the 

 ravages of its enemy. 



Here we may mention that all these molluscan shells, 

 like the eggs of birds, are formed, for the most part, of 

 calcium carbonate, the substance of which chalk is com- 

 posed. Sea and fresh-water molluscs derive the calcium 

 carbonate from the water in which they dwell ; land mol- 

 luscs obtain it from the vegetation which forms their food ; 

 and for this reason molluscs living on chalky soil have 

 heavier, stronger shells than those which frequent soils want- 

 ing in chalk. Related to the oyster are the file-shells, which 

 have the extraordinary habit, among molluscs, of building 

 nests. From their bodies a large number of very fine 

 threads are given off. These threads entangle themselves 

 with bits of shell, small stones and other flotsam and jetsam. 

 Beneath some favourable stone this ingenious nest is placed. 



Very many marine molluscs are burrowers, but their 

 work is of so simple a nature and displays so little 

 ingenuity that we cannot afford it more than passing 

 notice. Their burrowing simply consists of the animals 

 sinking themselves in mud or sand, that they may be 

 hidden from their enemies. The bodies of all molluscs 

 possessed of this habit are more or less elongated, so that 

 they are enabled to reach the water for feeding purposes, 

 while their shelly homes remain buried. Certain of these 

 molluscs do not even hide in the sand, but take up their 

 positions in the burrows of other marine organisms or in 

 cavities in the roots of seaweeds. On the other hand, 

 a few of these creatures are such inveterate burrowers and 



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