218 THE AQUARIAN NATURALIST. 



secreted, the interior of the shell being filled with 

 sea-water, in which the viscera are loosely suspended. 

 But a second and more important reason for the 

 employment of so many pieces in the construction of 

 this wonderful fabric is to be derived from examining 

 the mode in which the animal grows. Were it to 

 retain the same dimensions throughout the whole 

 period of its life, or could it at stated intervals cast 

 off its old investment and secrete a new and more 

 capacious covering as growth rendered the change 

 necessary, a simple earthy crust would have been 

 sufficient, without the presence of such an immense 

 number of joinings and sutures. The calcareous 

 plates of the Echinus, it must be remembered, are 

 merely secreted from the soft parts, having no vital 

 action going on within them, whereby, as in the bones 

 forming the skeletons of the higher animals, a con- 

 tinued deposition of fresh particles could be effected, 

 allowing of extension by interstitial deposit. How 

 therefore is the gradual expansion of the entire shell, 

 thus composed of a dense, dead crust, to be effected, 

 and that without ever deranging the proportions of 

 the whole fabric or necessitating a loosening of its 

 parts ? No other contrivance could apparently have 

 been adequate to the purpose ; nevertheless, by the 

 structure adopted, we see how admirably the growth 

 of the Echinus proceeds in all directions; for the 

 living and vascular membrane which covers the whole 

 external surface of the body, and dips down between 

 the edges of the various calcareous pieces, continually 

 deposits around the margin of each individual plate 

 successive layers of earthy particles, which, assuming 



