AMONG THE SEA-URCHINS. 



11 



plate. As the confined creature feeds and grows in bulk, the 

 pressure within would widen the seams, but each plate is gradually 

 increased in size by the deposition of carbonate of lime round its 

 edges, which thus gradually enlarge, increasing the bulk of the 

 whole shell in an exactly equal ratio. There are 20 rows of these 

 plates ranged in 10 alternate zones {a and /), with avenues of 

 pores between (see woodcut, Fig. i., A and B). On each plate is 

 seen the bosses or nipples, on which are fixed beautiful fluted or 

 sculptured spines (E), which lit on these nipples, on the ball-and- 

 socket joint principle, giving each spine a power of rotation in every 

 direction. Each large spine is surrounded by smaller ones, while 

 from the double row of pores proceed the ambulacra or tube feet. 

 Forbes calculates that a full-sized urchin will carry on its surface at 

 least 4,000 spines, while there will be 1,860 tube feet, each 

 occupying two of the tiny holes. 



^'m^ 



Fig. I. Morphology of Sea-Urchin. 



It is really wonderful, when carrying on a series of experiments 

 with these animals, to watch the co-ordination of action throughout 

 the muscular coat, which lines both the outside and inside of the 



