1.90 THE MECHANICAL FUNCTIONS. 



ing from her usual plan ; first, by dividing the 

 shell of the Echinus into a great number of small 

 pieces ; and secondly, by giving to each piece the 

 polygonal form, which is best adapted to their 

 mutual and perfect junction, without leaving any 

 intervening spaces. Thus has she provided for the 

 enlargement of the whole structure, by admitting of 

 additions being made to the margins of each of the 

 separate polygonal pieces ; fresh layers of cal- 

 careous substance being deposited on the under 

 side, and on the edges of each, in proportion as the 

 expansion of the contents of the shell causes their 

 separation. That such a succession of deposits has 

 taken place, may easily be seen, by minutely exa- 

 mining the texture of the plates, which will be found 

 marked by concentric polygonal lines. (Fig. 99.) 



The spines of the Echinus must be formed by 

 the successive deposition of layers on their outer 

 surface, as appears from the examination of their 

 structure, when a longitudinal section of them has 

 been made. The lines exhibiting the succession 

 of layers are seen in Fig. 100, which represents 

 such a section. Hence they are probably deposited 

 by the membrane which covers them during the 

 whole period of their growth. 



There is probably no series of animals that ex- 

 emplify in so marked a manner as the Echinoder- 

 mata the gradations which nature has observed in 

 passing from one model of construction to another 

 of a totally different aspect, through every inter- 

 mediate form. What shapes can be more diversi- 

 fied, and apparently irreducible to a common 

 standard, than those of the star-like Asterias, (Fig. 

 88) of the globidar Echinus, (Fig. 91) and of the 

 lily-shaped Pentncrimis; (Fig. 94) and yet we find 



