ECHINODERMATA. 57 



In such species as these, where the vital parts 

 are inclosed in a box of hard shelly substance, 

 there is a beautiful provision for progressive 

 growth. The shell is composed of a multitude of 

 plates (nearly 600 in all) of regular angular forms, 

 all dove-tailed together with the greatest nicety, 

 each enveloped in a very thin layer of living flesh 

 which secretes and deposits calcareous matter. 

 Now, if the shell had been made in one piece, 

 formed as it is by the deposition of lime from the 

 surface of the animal, it is manifest that every 

 layer deposited would have diminished the interior, 

 while the growing animal would ever be requiring 

 more room. But as it is, every one of these 

 angular plates is increased by layers on its inner 

 surface, each layer being a little wider than the 

 preceding ; and thus each piece gradually enlarges, 

 (and therefore the bulk of the whole shell also,) 

 while at the same time the definite form of every 

 one is accurately maintained. These 600 plates 

 bear on their surface above 4,000 spines, each an 

 exquisite structure, formed of minute chambers 

 separated by thin calcareous walls, transparent as 

 glass, often beautifully fluted or otherwise sculp- 

 tured, witli a ball and socket-joint at its base, and 

 muscles which give it motion. Well may the 

 eloquent historian of the British Starfishes observe, 

 that " the skill of the Great Architect of Nature is 

 not less displayed in the construction of a Sea- 

 urchin than in the building up of a world ! " * 



Among the spines and suckers of a Sea-urchin 



we discover a multitude of other objects endowed 



with vigorous motion, and highly curious in their 



nature. For a long time it was a matter of un- 



* " British Starfishes," p. 153. 



