SHORES OF THE CLYDE AND FIRTH. 
59 
coloured gelatinous mass. Tracing still further down the 
interior, from a close and careful study we discover in the 
centre of the bulb or root of each branch a little globular 
sack also filled with a gelatinous substance, but presenting 
to the eye more of the colour of the limy material of the 
spines and shell, and these sacks are each furnished with a 
little tube leading into the interior of the larger one below. 
Into the large tube itself we next turn our gaze, and at 
once observe the interior dotted over with little crimson 
spots, which are evidently corpuscles, but the colour of 
these as they approach the skin change into a darker tint 
interspersed with an orange hue. From these observations, 
and our knowledge of the construction of these organs, it 
is plain that in their sweeping gyrations and closing up 
process, the object is to fish or gather in the nervous 
matter required by the skin, of which, when the sacks are 
full, by bringing the branches together or nearly so, the 
interior skin, pressing against the sacks acting like a force- 
pump, send the corpuscles and fluid substance down the 
tube into the skin below. 
In the second place. What are the uses of the skin? 
Having traced the source from whence the fleshy tissue 
receives its life-sustaining food, it is again plain that its 
work is simply to supply the working movements of the 
pedicellariae and spines, and maintain its own existence 
between the segments, having not the power necessary in 
itself to force the expansion of the ball, and send deposits 
beyond the surface of the shell. That the skin, however, 
is a storehouse of great vitality, I have no doubt. Recently 
I cut a living urchin into sections, after cleaning out the 
interior of one section, leaving not a particle of matter 
on the shell. I deposited it in a vessel of salt water, 
and soon saw the spines and pedicellariae in active motion; 
