Star-fishes, Sea-urchins, etc. 



747 



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-caUed " 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. 





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LOXG-SPIXED SEA-URCHIXS. 



The Eeedle-Iike spines of these sea-urchins are over a foot in length. Acres of these creatures may be sometimes seen on tidally exposed areas 

 of the Queensland Great Barrier Reef, where this photograph was 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, fi\'e 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 



