THE SEA-URCHIN 39 



Jurassic rocks the pygaster being possibly the best 

 known. 



All the sea-urchins have an arrangement of tubes 

 and canals, known as the water-vascular system, 

 through which water is conveyed to all parts of their 

 organization. Starfishes and kindred creatures are 

 similarly endowed. A hidden beauty connected 

 with most members of this family is the ' madrepore 

 plate' or ' madreporite.' Notwithstanding its name, 

 it has nothing to do with coral structure. To see 

 this beautiful object to perfection it will be necessary 

 to take up a sea-egg or sea-urchin, or better still, a 

 starfish, and with a pocket magnifier to examine the 

 upper or dorsal side of the creature, when a white 

 spot will be observed near the centre. This will be 

 found to bear all the appearances of a piece of white 

 coral. The sea water is said to be admitted through 

 this sieve, the entrance to a branch tube which com- 

 municates with the water-canal system. Its sup- 

 posed duty is to keep out particles of sand and other 

 small objects that would interfere with the creature's 

 comfort. Thus in the madrepore plate utility is 

 combined with hidden beauty. 



A student possessing a microscope and living near 

 any sea-shore will always have facilities for studying 

 some representative of the great family of sea- 

 urchins. In addition to cutting sections of the 

 spikes for the purpose of seeing the incomparable 

 designs displayed, as already alluded to, if a portion 

 of a spike be examined while the creature is alive, 



