56 
ANIMAL LIFE ON THE 
no means small portion of the shell. Let us turn to the 
spines. Were they placed there by the Designer for the 
sole purpose of adorning still further what he had already 
made handsome in colour and complete in form at every 
point ? While Nature is lavishly profuse in adornments, 
and forgets not the meanest of her works, she wastes no 
time in the production of useless designs and idle forms. 
The spines have their work to do. Gosse, while uttering 
not a word on the point himself, attacks Profe sor 
Agassiz for supposing them to be organs of locomotion, and 
wonders whether the Professor ever saw a living urchin in 
motion. Had he, as he should have done, " looked before 
he leaped," he would have found that Agassiz was 
partly right, for under certain circumstances the spines 
are used in locomotion. If, for instance, the creature i-j 
cast upon a sand bank, as is often the case, each sucker 
axm can only lay hold of an atom of sand which has no 
holding power to sustain the dragging strain required, and 
in such a contingency, if the spines could not stilt the 
creature along, it would be left completely helpless. The 
rollicking motion observable in these appendages is due to 
their construction at their base, as seen in accompanying 
wood-cut. The interior is scooped out like a saucer, in the 
centre of which a cavity dips to the form and dimensions of 
a tubercle or round pivot raised upon the surface of the 
shell, on which it sits and works like a jointed ball socket, 
being held in its position by the investing fleshy tissue. 
The edges fitting the round of the shell travel like a wheel 
in a considerably extended circuit round each pivot, thus 
keeping free from the touch of the tissue more than half 
the entire shell. Up the spines again the tissue only 
reaches to something less than a fifth of the whole height. 
Nature is no bungler ; her unalterable physiological laws 
