?0 NATURAL HISTORY OP OUR 



The sea-urchins are more or less globular in shape, 

 although some families, as we shall see, are roughly heart- 



The French naturalists define them as " Our sins reguliers" 

 and " Oursins irreguliers " (compare Figs. 29 and 31). 



Instead of a leathery plate or spine-protected envelope 

 they have a shelly test, built up of a large number of plates, 

 which fit together at the edges in a most beautiful, and 

 always regular, manner and uniform pattern. The five- 

 radiate arrangement is only apparent outwardly by the 

 ambulacral grooves, as stated. 



The alimentary canal, and the ducts of the reproductive 

 glands, open, as in the star-fishes, at the top. The shelly 

 plates which edge the grooves are perforated for the ex- 

 trusion of the tube feet. By these little openings, as well 

 as at base and apex, connection exists between the internal 

 anatomy and a thin membrane which envelops the test. 

 On the shelly plates of the test are arranged in regular order 

 a large number of little protuberances or knobs (see Fig. 28), 

 and these form the base for the sharp spines which radiate 

 on all sides. The spines, which are so well attached that 

 they appear to form a portion of the test, are not so rigidly 

 fixed as they appear, but are movable at the will of their 

 owner, the little knobs and the bases of the spines forming, 

 with the membrane between, a sort of ball-and-socket joint. 



Small and curiously shaped three- pointed beaks, like 

 pincers with three jaws, set upon short stalks, occur be- 

 tween the spines, chiefly on the under side near the mouth. 

 These beaks open and shut with a snapping movement. 

 They are termed pedicillaria, and their function is not 

 well known. 



These pedicillarece exist in the star-fishes as well. 



Projectible through a large opening underneath the test 

 are five large teeth, with sharp, enamel-pointed|tips. The 

 teeth are made up of a number of plates, and the set is con- 



