34 OCEAN LIFE. 



opportunity occurs, though here, numbers are of little conse- 

 quence. Let us rather ask why Nature in this case has chosen to 

 work by laws so complex, when we might suppose a simpler fabric 

 might have done as well ? why frame a shell composed of thou- 

 sands of small portions thus connected, when even in higher 

 animals all this elaborate device can be dispensed with ? A little 

 thought will convince us that, even here as elsewhere, Nature 

 has employed no useless superfluity of structure. To case the 

 animal in stone, would have been a simple process ; as we have 

 already seen that almost all the zoophytes secrete calcareous 

 matter in abundance ; but, when thus closed up in a stony shell, 

 how could the creature grow ? Here is the difficulty. We might 

 as well expect a Chinese lady's foot to grow while shod in iron, 

 as that an Echinus should expand from the small size it first 

 presents, to its adult dimensions, without some remarkable pro- 

 vision being made to allow of such an increase. Growth con- 

 stantly goes on, and yet the crust itself, being, when once formed, 

 devoid of life, and as incapable of growing as if made of marble, 

 cannot expand as do the bones of higher animals, neither is any 

 part left soft, but at all ages, the whole shell has the same com- 

 pactness and solidity throughout, and presents the same precise 

 distinctive form peculiar to the species. The only way in which 

 this difficulty could be met is, obviously, that which Nature has 

 adopted. 



" The shell is made of numerous pieces, between the contiguous 

 margin of which, the shell secreting tegument dips down, adding 

 continually to the circumference of every piece, cretaceous parti- 

 cles, layer after layer, by which the superficial size of each pro- 

 gressively increases, whilst its form remains unchanged. The 

 thousand pieces that compose the shell, thus simultaneously be- 

 come enlarged, and as they never change their figure or the 

 relative proportions that they bear to all the rest, the entire 

 shell expands without the slightest deviation from the given form, 

 till it attains the limit of its growth. 



" Examine well the spines. The base of every spine presents 

 a smooth concavity or socket, which exactly fits one of the 

 rounded tubercles already pointed out, upon the outer surface of 

 the shell, and forms a perfect joint. No matter how minute the 



