STAR-FISHES, SEA-URCHINS, ETC, 



349 



on a definitely sj'mmetrical 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, collectiveh-, the urchin is able to adhere to and travel o\er 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 Hatter 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. 





_ . .*.. 









LONG-SPINED SEA-URCHI\S 



TAf neeiHe-like spir.es of theu se-a-urcfiins an oi'er a foot in length. Acres of these creatures may he sometimes seer, on tidally exposed 

 areas of the ^hteensland Great Barrier Reefy ivhere this photograph luas taken 



The membranous disk \\hich co\-crs the upper and smaller circular aperture in the living 

 animal is perforated centrall}- b\- the \ent, and around it are grouped the eye-spots and sundry 

 excretory apertures. 



A noteworth)- 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 

 23 



