STARFISH, SEA URCHIN, BRITTLE STARS 



96 



to the tips, and it is difficult to tell where the disk stops 

 (Fig. 46). 



Brittle stars. — These are starfish that are at once dis- 

 tinguished from those above, by the long, slender, tapering, 

 and cylindrical arms. The central disk is also more sharply 

 marked off from the arms (Fig. 47). Again, there are no 

 grooves along the oral 

 surfaces of the arms, and 

 the tube feet protrude 

 througli the sides of the 

 arms instead of through 

 the ventral walls, and are 

 not provided with suck- 

 ers. Hence, locomotion 

 is effected largely by the 

 slender arms. In some 

 species of brittle stars, 

 the basket stars, the arms 

 are branched many times, 

 producing a complex but 

 beautiful object. Some 

 brittle stars have the remarkable property of being able 

 to throw off pieces of their arms when touched or irri- 

 '. tated; hence the name, brittle stars. 



Sea urchins. — Sea urchins are common along the eastern 

 coast of the United States, especially where the shore is 

 rocky. They are globular in shape, but mth the side, on 

 which the mouth is situated, flattened. These echinoderms 

 have no distinct arms, but the body is bristling with cylin- 

 drical, pointed spines jointed to the skeleton so that they are 

 movable (Fig. 48). There are also projecting among the 

 spines, five double rows of tube feet radiating from the 



Fig. 47. — Brittle starfish. Note that 

 the disk is clearly defined. 



