THE SEA-URCH1X. 137 



The calcareous shell of the sea-urchin seems at first sight to 

 be composed of one simple crust, but proves on nearer inspec- 

 tion to be a beautiful piece of mosaic, consisting of several 

 hundred parts, mostly five-sided, transversely oblong, and dis- 

 posed in twenty vertical rays or columns. Ten of these are 

 narrower, and consist of smaller pieces, which are perforated 

 with holes for the feet or suckers ; they are thence termed ani- 

 bulacral : the other ten are broader, and consist of larger pieces. 

 The ten ambulacral columns are disposed in five pairs, with 

 which the ten larger columns, also disposed in pairs, alternate. 

 The number of plates in a row varies with the age of the ani- 

 mal, increasing as it grows older and larger. They are marked 



Shell of Echinus, or Sea-Urchin. 



on the outside with tubercles or knobs of various sizes, which 

 support the spines. The spines themselves have a cup-like 

 cavity at their base, which is connected with, and moves on, the 

 prominent tubercle. The pieces of which the shell is composed 

 are so closely united that their junctions are hardly visible, but 

 on allowing the shell to macerate for some days in fresh water, 

 it falls to pieces. This complicated structure is by no means a 

 mere architectural luxury, a useless exuberance of ornament, 

 but essentially necessary to the requirements of the animal's 

 growth. A simple hard crust would not have been capable of 

 distension, whereas a complicated shell, such as the sea-urchin 

 possesses, can grow in the same ratio as the internal parts, by 

 continual deposits on the edges of the individual pieces. On 

 closely examining a living sea-urchin, we find the whole surface 



