WONDERFUL COMPLEXITY OF STRUCTURE. 217 



It is impossible, by any verbal description at all 

 commensurate with our present undertaking, ade- 

 quately to explain the more minute contrivances visi- 

 ble in the disposition of every portion of these won- 

 derfully constructed coverings. It is sufficient for 

 our present purpose to observe, that the globular crust 

 of an Echinus is made up of several hundred poly- 

 gonal pieces of different sizes, and, although pre- 

 senting every variety of outline, generally approxi- 

 mating 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 can scarcely be 

 distinguished, even upon the most minute examina- 

 tion ; and that from the union of so many distinct 

 and dissimilar plates results a firm, compact and 

 beautiful box, the shell of the " Sea-Egg." 



The first question that naturally suggests itself on 

 examining a shell of this description, is concerning 

 the object to be attained by such remarkable com- 

 plexity. It would appear, at first sight, that a simple 

 calcareous crust, like the integument of a lobster, had 

 it been allowed to exude from the entire surface of 

 the Echinus, would gradually have moulded itself 

 upon the body of the creature, and thus have formed 

 a globular shell, composed of but a single piece, but 

 answering every purpose connected either with sup- 

 port or defence. 



A very little investigation, however, will serve to 

 show the necessity for the elaborate arrangement 

 here exhibited. In the first place, the earthy matter 

 is not deposited upon the surface of the body, but 

 within the soft external integument by which it is 



