210 THE MECHANICAL FUNCTIONS. 



portion as the expansion of the contents of the 

 shell causes their separation. That such a suc- 

 cession of deposits has taken place, may easily 

 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 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 pro- 

 bably 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 Echino- 

 dermata, the gradations which nature has ob- 

 served in passing from one model of construction 

 to another of a totally different aspect, through 

 every intermediate form. What shapes can be 

 more diversified, and apparently irreducible to a 

 common standard, than those of the star-like 

 Asterias, (Fig. 88) of the globular 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, assuming gradually 



