SHELL OF THE ECHINI. 191 



portion of the shell : the entire series of pieces in each row resembles in 

 figure the shape of the space included between two of the lines mark- 

 ing the degrees of longitude on a terrestrial globe broad at the equator, 

 but gradually narrowing as it approaches the poles, an arrangement, 

 of course, rendered necessary by the spherical form of the creature. 

 There are ten rows of these tuberculated plates ; but as they are dis- 

 posed in pairs, each row of large pieces being united by a zigzag suture 

 with another of a similar description, there are in reality only five large 

 segments of the shell, each supporting a double row of tubercles. 



(502.) The reader must not, however, conclude that the great central 

 tubercles above mentioned are the only parts of the shell to which spines 

 are affixed ; hundreds of smaller elevations are disseminated over the 

 surface, whereunto smaller spicula are appended although, from their 

 diminutive size, these are of secondary importance in locomotion. 



(503.) The five large double segments that thus form the greater 

 portion of the calcareous shell are separated from each other by the 

 interposition of ten rows of perforated plates, likewise disposed in pairs, 

 and composed of much smaller pieces than those which support the 

 tubercles ; hundreds of foramina, piercing these ambulacra! bands, give 

 passage to as many tubular feet or protrusible suckers, in every respect 

 resembling those of Asterias, and distended by a similar apparatus. 



(504.) It is impossible, by any verbal description at all commensurate 

 with the limits of our present undertaking, adequately to explain 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 pre- 

 senting 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 uniting 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, com- 

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

 first question that naturally suggests itself, on examining a shell of 

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

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

 simple calcareous crust, had it been allowed to exude from the en- 

 tire surface of the Echinus, would gradually have moulded itself upon 

 the body of the creature, and thus have formed a globular shell without 

 suture, answering every purpose connected either with support or de- 

 fence. 



(505.) 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 



