2G6 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



closely resembles sea-water, but is, nevertheless, 

 richly corpusculated ; it coagulates when exposed 

 to the air, and otherwise shows that it is something 

 more than mere sea- water. If we look closely into 

 the shell which has been deprived of its spines, we 

 find that it is composed of a great number of small 

 hexagonal j)lates (Fig. 41), the edges of which fit £0 

 closely together that the whole shell is converted 

 into a box, Avhich, when the animal is alive, is 

 water-tight, as we have proved by submitting the 

 contained fluid to hydrostatic pressure, under which 





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Fig. 41.— A portion of the external shell of an Echinus denuned of ppines and 

 slightly magnified, showing the arrangement of the plates, the balls in the ball- 

 and-socket joints of the spines, and the holes through which the ambulacral feet 

 are protruded. (From Cas.-ell's "Nat. Hist.") 



circumstances there is no leakage until the pressui-e 

 is sufficient to burst the shell Nevertheless, if we 

 look closely at the dned shell of an Echinus, we 

 shall .see that it is not an absolutely closed box ; for 

 we shall see that the hexagonal plates are so arranged 

 as to give rise to five double rows of holes or pores 

 (Fig. 41), which extend symmetrically from pole to 

 pole of the animal (Fig. 39). It is through these 

 holes that the tube-feet are protruded ; so that if 

 we imagine a pentagonal species of Star-fish to be 

 curved into the shape of a hollow spheroid, and 

 then converted into a calcareous box with holes 



