ECHINUS. 209 



size as that which they are intended to have 

 at all future periods ; because, as we have just 

 seen, after the innermost layer has been depo- 

 sited, they admit not of any future enlargement 

 of their cavity. Thus we find that, in the case 

 of polypes which are lodged in cells, the walls 

 of these cells must be completed before the soft 

 polypous portion has attained its full expansion; 

 for were it at first built of a smaller size, propor- 

 tioned to that of the young polype, it would 

 prevent all farther growth. 



The globular shell of the Echinus, which is 

 external to the soft parts that nourish it, and 

 which yet grows from a very minute sphere 

 to one of large dimensions, keeping pace with 

 the gradual expansion of the internal organs, 

 might appear to be an exception to the general 

 law. Nature has, however, accomplished her 

 purpose without deviating 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 per- 

 fect junction, without leaving any intervening 

 spaces. Thus has she provided for the enlarge- 

 ment 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 being deposited on the 

 under side, and on the edges of each, in pro- 



VOL. i. P 



