146 OUR COMMON BRITISH FOSSILS. 



of these five canals gives off in its course innumerable 

 tubes, which protrude through the little punctures 

 and lengthen at the will of the animal. At the 

 base of each little water-tube is a little water-bag, 

 and when this is compressed (as a boy squeezes 

 a hollow indiarubber ball he has filled with water) 



the minute water-tubes, 

 or " ambulacral feet," 

 are lengthened even be- 

 yond the spines of the 

 animal. Myriads of them 

 can thus be protruded 

 whenever the sea-urchin 

 thinks fit, and they may 

 then be seen wriggling 

 and moving about like 



Fig. 120. — Test or shell of Cidaris coronata, 



showing the tubercles to which the bases sO manV WOTmS aS WC 

 of spines are attached (Oolitic formation). ^ ' 



saw was the case with a 

 turned-over star-fish. At the tip of each is a sucker, 

 and thus, when a few scores of the " ambulacra " are 

 thrust forth, and have attached themselves to any 

 object, they are enabled to warp the entire shell along. 

 It is in this way, in fact, that most of the true Echi- 

 nodermata crawl along the bottom of the sea. The 

 reverent reader cannot fail to be struck with such 

 a beautiful piece of construction, and a hint might here 

 be furnished to our hydraulic engineers. That this 

 principle has been in vogue for myriads of years is 

 evident by the similar construction of the ancient Sea- 



