162 ECHINODERMATA. 



the more minute contrivances visible in the disposition of every 

 portion of these wonderfully constructed coverings : it is sufficient 

 for our present purpose to observe that the globular crust of an 

 Echinus is made up of several hundred polygonal pieces of 

 different sizes, and, although presenting every variety of outline, 

 generally approximating more or less to a pentagonal form ; that 

 these pieces are so accurately and completely fitted to each other, 

 that the lines which unite them are scarcely to be distinguished 

 even upon the most minute examination ; and that from the union 

 of so many distinct and dissimilar plates results a firm, compact, and 

 beautiful box, similar to that represented in the figure. The first 

 question which naturally suggests itself on examining a shell of this 

 description, is concerning the object to be attained by such remark- 

 able complexity ; it would appear indeed, at first sight, that a simple 

 calcareous crust, had it been allowed to exude from the entire sur- 

 face of the Echinus, would gradually have moulded itself upon the 

 body of the creature, and thus have formed a globular shell with- 

 out suture, but answering every purpose connected either with 

 support or defence. 



(203.) A very little investigation, however, will suffice to show the 

 necessity for the elaborate arrangement to which we have alluded. 

 In the first place, as we shall immediately see, the earthy matter is 

 not deposited upon the surface of the body, but within the soft 

 external integument by which it is 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 em- 

 ployment of so many pieces in the construction of the shell of an 

 Echinus is to be derived from examining the mode in which the 

 animal grows ; was 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 sutures and joinings. 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, by which, 

 as in the bones forming the skeletons of vertebrate animals, a con- 

 tinual deposition of fresh particles could be effected, allowing of 

 extension by interstitial deposit. How, therefore, could the growth 

 of the animal be provided for ? How is the gradual expansion of 

 the entire shell, thus composed of a dense and extravascular crust, 



