1 68 AN INTRODUCTION TO THE STUDY OP FOSSILS 



columns of plates through which the long, sucker-bearing tube- 

 feet protrude (Fig. 67, A t.f.) ; the middle of the area is without 

 pores for tube-feet and bears numerous spines. The tube-feet 

 number about 1800, and can be protruded beyond the longest 



Fig. 67. — An enlargement of a small portion from Fig. 66. A, an enlargement of 

 the seventh and eighth interambulacral plates, and the adjoining ambulacral ones, 

 counting from the first ocular to the right of the madrepore. B, a single spine 

 ( X 5), showing its ball-and-socket joint and the attachment of the muscles to move 

 it. b., base of spine ; m., muscles to move the spine ; ped., the small grasping pedi- 

 cellariae with the minute pincer-like heads; sp., large and small spines; /./., tube- 

 feet ending in suckers and capable of expansion to double the length shown here, 

 or of contraction to far within the spines ; tu., tubercle. 



spines. Between each set of two ambulacra are the inter- 

 ambulacra, — broad, spine-bearing areas composed of two col- 

 umns of plates. The water-vascular system is similar to that of 

 the starfish, except that the radial canals giving off the tube-feet 

 lie within, not without, the test. 



Growth takes place by the increase in size of the plates and 

 by the addition of new plates dorsally at the ends of the ambu- 

 lacra and the interambulacra. 



A complicated apparatus, called Aristotle's lantern, is connected 



