STARFISHES, SEA-URCHINS, ETC. 



95 



recognized. Some starfish may have thirty or more rays 

 instead of five as in the typical forms. Others may have the 

 interradial spaces filled out nearly or quite to the tips of the 

 rays, making the animal simply a pentagonal disk. Some 

 are very small, less than an inch in diameter, while others 

 attain an extent of three feet or more. 



The brittle-stars belong to the class Ophiuroidea (Gr. ophis, 

 snake; our a, tail; eidos, likeness). They differ from the 



FIG. 35. A holothurian or sea-cucumber, Cncumar'm frondosa. (About 



| natural size.) 



Aster oidea in that the body-cavity does not extend out into 

 the arms as it does in the starfish, or extends at most only into 

 the bases of the arms. The five radiating rays are usually 

 slender, more or less cylindrical, and sometimes branched. 



The sea-urchins, class Echinoldea (Gr. echinos, hedgehog; 

 eidos, likeness), look quite unlike the starfish. The body is 



