76 STAR-FISHES, SEA-URCHINS, ETC. 
5, Fig. 2) is of a reddish-brown color, and com- 
pletely covered over with minute silken bristles. 
These, on the death of the animal, are rapidly re- 
moved, and then the flat disk can be plainly seen 
to be made up of a large number of closely-fitting 
polygonal plates, arranged in twenty series around 
the circumference. The petals of the central star, 
which now becomes visible, will be found to be 
made up of transverse slits, and if the direction of 
the petals is followed to the border of the disk, it 
will be seen that the slits are continued by pores. 
Through these slits and pores, which occupy five 
pairs of plates, the tube-feet, similar in character to 
those of the star-fish, pass out, and hence define 
ambulacral zones. The intermediate five. pairs of 
plates, from which pores are absent, will then be 
the interambulacral areas. Now, imagine the arms 
of astar-fish turned over the back of the animal so 
as to have the tips meet, the animal then flattened 
out, and the sides of the arms so expanded as to 
close in the interspaces: you would then have a 
construction much as in the sand-dollar and in other 
sea-urchins. This appears really to be the relation, 
but just how the diverging modification has been 
brought about still remains to be determined. The 
eye-specks, which in the star-fish are placed at the 
extremities of the arms, are in the urchin situated 
centrally on top, or just where we should expect to 
find them on the assumption above stated. 
Alternating with the eye-specks are the ovarian 
apertures, through which the eggs are passed out 
from the body. In the cluster of small plates which 
