Z 
266 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 plates (Fig. 41), the edges of which fit so 
closely together that the whole shell is converted 
into a box, which, when the animal is alive, is 
water-tight, as we have proved by submitting the 
contained fluid to hydrostatic pressure, under which 
Fig. 41.—A portion of the external shell of an Echtnus denuned of spines 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 Cassell’s *‘ Nat. Hist.’’) 
circumstances there is no leakage until the pressure 
is sufficient to burst the shell Nevertheless, if we 
look closely at the dried shell of an Kchinus, 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 
