136 



ECHINODERMATA 



pairs to each arm. " Brittle stars " have highly developed autotomy 

 and the ability to regenerate new arms. (Figure 58, ^^ and 5.) 



Fig. 58. A, serpent star. 5, sand dollar. (From Verrlll.) 



Class 3. Echinoidea. — The sea urchins are not star-shaped, but 

 globular. The shell or test is made up of firmly united ossicles 

 ranged in rows which run from the oral to the aboral poles. Many 

 of the plates bear movable spines which aid in locomotion. Five 



ambulacral plates have openings 

 into the egg sacs. There are five 

 bands of distensible locomotor 

 tube feet beginning near the oral 

 opening and running towards the 

 aboral pole. The distinctive 

 feature of the sea urchin known 

 as Aristotle^ s lantern consists of 

 five jaw-like structures, each 

 bearing a rather large sharp 

 white tooth. The intestine is 

 quite long and has no radiating 

 ceca. Sea urchins are able to 

 chisel out solid rock by turning 

 round and round. The spines 

 of large sea urchins have been 

 used as slate pencils by missionaries in the Pacific Islands. 



The sand dollars or " cake urchins'" are flattened and disc-like, 

 living near the surface of the sand. The heart urchins or " sea bears " 

 bury themselves in the muddy sands for several inches. Under the 



Fig. 59. Purple sea urchin. (From 

 Mayer, Seashore Life. Courtesy of N. 

 Y. Zool. Soc.) 



