CRAB CATERPILLARS 75 



in ponds and rivers, as well as in the sea, and 

 so well do they perform their task that both the 

 air and the water are always kept pure. 



Another very interesting fact about zoeas is 

 that they form the chief food of no less a creature 

 than the Greenland whale. No doubt you know 

 that whales are of two kinds those which have 

 teeth, and those which have none. Those which 

 have teeth feed upon fishes, and giant cuttles, 

 and could easily swallow a man. But the whales 

 which have no teeth have throats so small that 

 they would almost certainly be choked if they 

 tried to swallow a herring! So they have to 

 feed on very small creatures indeed, and are 

 very fond of zoeas, which often swim about in 

 such vast shoals that the water of the sea is 

 quite thick with them. And they catch them in 

 a most curious manner. 



You have heard, of course, of the very useful 

 substance which we call "whalebone;" and no 

 doubt you know that it has nothing to do with 

 the bones of the whale at all. It is found in the 

 mouths of those whales which have no teeth, 

 and hangs down in great plates from the gums 

 of their upper jaws. Very soon these plates split 

 up; and then each part splits up again; and so 

 on, over and over again, till at their lower ends 

 they form a kind of thick fringe of close, matted 

 hairs. 



Now it is by means of this fringe that the 



