90 STRUCTURAL AND SYSTEMATIC ZOOLOGY 



About two hundred and fifty species are known. The 

 common starfish (Asterias) has four rows of feet in each 

 ray. Solaster, the "sun star," has numerous rays with 

 two rows of feet in each. Goniaster is somewhat pen- 

 tagonal in form with feet arranged as in the " sun star." 

 Asteroidea are found as fossils. 



CLASS 2. Ophiuroidea 



These are star-shaped echinoderms with a central 

 disk and five flexible, jointed arms distinctly marked off 



FIG. 47. Ophiocoma russet, an Ophiuran; natural size. West Indies. 



from the disk, the latter containing all the visceral parts. 

 There is no anal opening and the madreporite is on the 



