SHELL OF ECHINU8. 



137 



structure as the rays themselves, and then we 

 have a complete Echinus, complete, at all events, 

 as to its general external appearance. 



The specimens that are found cast up by the 

 waves are generally destitute of the spiny arma- 

 ture that is found upon them in their living 

 state, and thus permit the eye to perceive the 

 formation of the shell. Close by the figure of 

 the Echinus itself will be seen a little diagram 

 composed of several angular forms studded with 

 little tubercles. These represent the pentagonal 

 plates of which the shell consists, and which are 

 most wonderful instances of animal economy. 

 In the shell of every Echinus are hundreds of 

 these plates, varying in size according to their 

 position, and so closely connected with each 

 other that externally their marks of junction 

 are not perceptible; but if the shell is broken, 

 and examined from the interior, the shape of 

 these plates becomes tolerably well defined. It 

 will be observed also, that when the shell is 

 broken, the serrated edges of the fractured 

 portions show the angular form of the plates. As 

 the shell is composed of these plates, it may well 

 be asked how the creature can possibly increase 

 in size, because it cannot, like the lobster and 

 other crustaceans, throw off its old coat when too 



