STAR-FISHES, SEA-URCHINS, ETC. 
349 
on a definitely symmetrical pattern with minute perforations, such perforations being most 
distinctly visible on the inner surface of the shell. These minute punctures are the 
apertures through which in life the delicate tubular locomotive organs, or so-called “ feet,” 
are thrust out and retracted. The majority of these tubular organs terminate in a circular 
sucking-disk, wherewith, collectively, the urchin is able to adhere to and travel over the 
surface of the smoothest rock, or even up the glass walls of an aquarium. In the empty 
beach-gathered urchin-shell a circular hole may be observed at the two opposite poles, the 
one in the centre of the lower and flatter surface being the larger of the two. It is within 
this lower and larger one that the mouth, with its complex apparatus of teeth, is suspended. 
r 
Photo by IV. Savillt^Kent^ F.Z.S. 
LONG-SPINED SEA-URCHINS 
The needle-like spines of these sea-urchins are o'ver a foot in length. Acres of these creatures may he sometimes seen on tidally exposed 
areas of the ^lueensland Great Barrier Reef ivhere this photograph 'zuas taken 
The membranous disk which covers the upper and smaller circular aperture in the living 
animal is perforated centrally by the vent, and around it are grouped the eye-spots and sundry 
excretory apertures. 
A noteworthy feature associated with the greater portion of the structural details of the 
sea-urchin which have been enumerated is the dominance of the number five in the constituent 
elements. It is found, for instance, that the perforated areas through which the tube-feet are 
protruded form, as with the petals and other elements of many flowers, five symmetrically 
corresponding segments. The dental apparatus comprises five equivalent tooth-like structures, 
and there are five eye-spots and five excretory apertures at the upper pole. This particular 
number, with multiples of the same, is furthermore characteristic of all the typical members 
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