SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 137 



More obvious than mouth and teeth are usually the long 

 slender tube-feet, which form five double bands over the 

 test, and can be stretched out to a great length. They, 

 indeed, give the sea-urchin a great part of its beauty, and in 

 life are in constant movement, now extended, now con- 

 tracted. By this means the sea-urchin is enabled to crawl 

 up a perpendicular surface. The only other point which 

 can be readily observed in the living urchin is the posterior 

 opening of the food canal at the point opposite to the mouth. 

 It is surrounded by small plates of lime, and, as these are 

 readily removed, is in consequence often represented by a 

 large hole in dried specimens. 



To study the composition of the urchin's test in detail we 

 must return to the dried specimens from which the spines 

 have been rubbed off. As already noticed, the mouth is 

 usually now represented only by a gaping hole, by which 

 the lantern has been shaken out. The anus may or may 

 not have lost its small plates, but around it will be seen ten 

 distinct plates, which mark out as many radii on the shell. 

 Five of these plates bear each a distinct round hole, which 

 is the opening of the reproductive duct, but one of the five 

 is in addition perforated by minute holes, and so constitutes 

 the madreporite. The other five plates are smaller, and 

 bear each an eye-spot. In a line with these five plates 

 are the five ambulacral areas of the test, which each 

 consist of two rows of plates perforated by the minute 

 pores through which the tube-feet emerge. In addition 

 these plates, which are relatively narrow, bear a few spines. 

 Corresponding to the larger plates, and thus alternating 

 with the ambulacral areas, are five interambulacral areas, 

 each consisting of a double row of wide plates, bearing 

 numerous spines. The net result is to produce in the living 

 urchin five double rows of tube-feet, separated from each 

 other by a somewhat wide interval thickly covered with 

 spines. The spines have a curious ball-and-socket joint at 

 the base, and are very freely movable. They assist in 

 locomotion, and must also protect the test from mechani- 

 cal injury. The large urchin lives freely exposed, and 

 probably from its strong armour has little to fear from 

 the attacks of enemies ; but the little purple-tipped urchin 

 covers itself with weed and fragments of stone and shell as 



