166 ANARTHROPODA 



pronounced spines and usually a globular or disk-shaped 

 body without arms. The true urchins are common along 

 muddy coasts, and the sand dollars flat and with short, 

 weak spines are most prevalent along sandy shores. 

 The sea cucumbers (Hokthuroidea) have elongated cylin- 



FlG. 192. A brittle star (Ophiopholis bellis). Photograph natural size. 



drical bodies with a circle of branched tentacles about the 

 mouth, and from it five rows of feet extend posteriorly. 

 They are found in the mud at the water's edge during low 

 tide. 



The brittle stars (Ophiuroidea) have five arms much more 

 slender than those of the starfish and no ambulacral grooves 

 are present. The crinoids (Crincridea) including the feather 



