154 THE MECHANICAL FUNCTIONS. 



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 calcareous substance be^jng deposited on the under 

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

 sion of the contents of the shell causes their separation. 

 That such a succession of deposites has taken place, may easi- 

 ly be seen, by minutely examining 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 succes- 

 sive 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 exemplify in 

 so marked a manner as the Echinodermata, the gradations 

 which nature has observed in passing from one model of 

 construction to another of a totally different aspect, through 

 every intermediate form. What shapes can be more diver- 

 sified, and apparently irreducible to a common standard, 

 than those of the star-like Asterias, (Fig. SS) of the globu- 

 lar Echinus, (Fig. 91,) and of the lily-shaped Pentacrinus; 

 (Fig. 94,) and yet we find these passing the one into the 

 other by the most gradual transitions? Setting out from 

 the star with five slender rays, which is the standard form 

 of the Asterias; we find the rays, in succeeding species, as- 

 suming gradually a greater breadth at their base, and their 

 sides joining at more obtuse angles: the star-like form is 

 gradually effaced, and the outline is rather a pentagon, with 

 its sides curved inwards (Fig. 89.) We soon perceive this 

 curvature giving place to a straight line, so that the shape 

 becomes an exact pentagon. The next change effected is in 

 the angles of this pentagon, which by degrees are lost in a 



