BRANCH ECHINODERMATA : STARFISHES, ETC. 123 



drical. The distinguishing difference between the brittle- 

 stars and the starfishes is that the body-cavity and the 

 stomach which extend out into the arms in the star- 

 fishes are in the brittle-stars limited to the central disc, or 

 to the disc and bases of the arms. The tube-feet also 

 have no suckers at the tips. More than 700 species of 

 brittle-stars are known. They feed on marirjfe shell-fish, 

 crabs and worms. /' 



Sea-urchins (Echinoidea). The sea-urchins (figs. 20, 2 1 

 and 22) of which more than 300 species are known, have 

 no arms or rays, and they are usually not flat like the star- 

 fishes but globular, with poles more or less flattened. As 

 has been noted in the examination of the body-wall or 

 * ' shell, ' ' the radiate character of the body is shown by the 

 five radiating zones of tube-feet. The mouth, with its five 

 strong "teeth," is on the ventral surface, and the anal 

 opening and madreporic opening are on the dorsal sur- 

 face. The calcareous plates (seen distinctly in a specimen 

 from which the spines have been removed) which consti- 

 tute the firm part of the body- wall, are more or less 

 pentagonal in shape and are usually firmly united at the 

 edges. The spines which are so characteristic of the 

 sea-urchins vary much in size and number and firmness, 

 but are present in some form on all of them. 



\Yhile most of the sea-urchins live near the shore, 

 being very common in tide-pools, some live only on the 

 bottom of the ocean at great depths. Their food consists 

 of small marine animals and of bits of organic matter 

 which they collect from the sand and debris of the ocean 

 floor. Many of the sea-urchins are gregarious, living 

 together in great numbers. Some have the habit of 

 boring into the rocks of the shore between tide-lines. I 

 have seen thousands of small beautifully colored purple 

 sea-urchins lying each in a spherical pit or hole in hard 

 conglomerate rock on the California coast. How they 



