284 ELEMENTS OF COMPARATIVE ZOOLOGY. 



Usually in preserved urchins the stone-canal becomes so 

 tender as to be easily destroyed. It goes downward from 

 the madreporite to the inner end of Aristotle's lantern, 

 where it connects with a ring-canal, and from this arise 

 the radial canal, in much the same way as in starfishes, the 

 whole forming a water- vascular system. As in the star- 

 fish, the nervous system follows this water- vascular system. 



COMPAEISONS. 



With columns for starfish and sea-urchin, answer the 

 following questions : 



(1) What is the general shape of the body ? 



(2) Are the radial canals inside or outside the hard body- 

 wall ? 



(3) Do you find branchiae ? 



(4) Are all the spines movable ? 



(5) Is an Aristotle's lantern present ? 



(6) How many divisions to the reproductive organs ? 



(7) Are hepatic caeca present ? 



(8) Do you find a branchial tree ? 



(9) Do you find gastric pouches ? 



