STAR-FISH AND SEA-URCHINS. 265 
which occur in the Star-fish, though considerably 
larger, and the ambulacral system is constructed 
Fig. 39—An Echinus, partly denuded of its spines. (From Cassell’s ‘* Nat. 
Hist.”) 
upon the same plan. If we shave off the spines 
and pedicellariz (Fig. 39), we find that we come to 
Fig. 40.—Shewing interior of Echinus shell. (From Cassell’s ‘‘ Nat. Hist.’’) 
a hard shell, which, if we break, we find to be 
hollow and filled with fluid (Fig. 40). The fluid 
