158 HALF AN HOUR WITH SEA-UKCHINS 



" test " as it is usually called, is composed of pieces 

 of carbonate of lime, of which there are no fewer 

 than six hundred that go to make up the whole, like 

 a piece of beautiful mosaic-work. And yet this 

 same shell, which may be two or three inches in 

 diameter, is fundamentally the same as that possessed 

 by it when it was not so large as a pea ! How, then, 

 has it grown to its present size ? When the process 

 is explained, our readers cannot refrain from an ex- 

 pression of wonder at this additional instance of the 

 Creator's wisdom. We mentioned the membrane that 

 lines the outside of the shell this has the power of 

 secreting carbonate of lime from the sea- water, and it 

 is somehow inserted between and around every one 

 of the six hundred pieces that make up the whole 

 structure. Hence each of these can only grow along 

 Us edges, by the addition of lime, and, as all grow 

 slowly alike, the shell thus gradually enlarges itself 

 to suit the requirements of the soft-bodied parts 

 within, and which it is called upon to protect. 



The physiological organization of the Echinoderms, 

 as these creatures are also called, in allusion to their 

 " spiny skins," is much higher than might at first 

 be supposed. The sea-urchin, for instance, has a 

 mouth and anus, an alimentary canal, and a distinct 

 nervous system. But by far the most singular part 

 of its organization is that termed the " water 

 vascular " system, by means of which all the Echino- 

 dermata are enabled to move about. Among the 

 rest of the plates which build up the solid shell, 



