672 



THE SEA-UKCHIN. 



are nearly two thousand suckers continually at work, some being pro- 

 truded, others relaxed, and others still feeling for a holding-place, the 

 progress of the creature is very regular and gliding, and hardly seems 

 to be produced by voluntary motion. 



We will now proceed to some examples of these curious beings. 



We first take a beautiful fiimily of this order, called Echinidse, be- 

 cause they are covered with spines like the quills of the hedgehog. 

 Popularly they are known by the name of Sea-urchins, or Sea- 

 Eggs. 



In all these curious beings the upper parts are protected by a kind 

 of shell, always more or less dome-shaped, but extremely variable in 



The Sea-urchin {Echinus sphcera). 



form. The shell is one of the most marvellous structures in the animal 

 kingdom, and the mechanical difficulties which are overcome in its for- 

 mation are of no ordinary kind. In the case of the common Sea-egg 

 the shell is nearly globular. Now, this shell increases in size with the 

 age of the animal ; and how a hollow spherical shell can increase reg- 

 ularly in size, not materially altering its shape, is a problem of extreme 

 difficulty. It is, however, solved in the following manner. The shell 

 is composed of a vast number of separate pieces, whose junction is evi- 

 dent when the interior of the shell is examined, but is almost entirely 

 hidden by the projections upon the outer surface. These pieces are of 



