STRUCTURE OF THE STARFISH. 63 



and unite, and project over the mouth, to form the mouth- 

 papiUrB. 



1. Carefully remove these spines, so as to expose the 

 inter-arabulacral plates, and notice that these approach and 

 unite to form the inter-nidial partitions. 



p. Make a sketch of the lower surface of the disc. 



X. THE STRUCTURE OF THE STARFISH. 



( Aster acantheon herylinus.) 

 INTERNAL ANAT03IY. 



I. In an alcoholic or a living specimen notice the fol- 

 lowing external organs : — 



a. The two double rows of tubular feet, or ambulacra, 

 which project from the ambulacral furrows, upon the oral 

 surface of each ray. 



b. The membraneous peristome which fills the space 

 between the mouth and the bases of the rays. 



c. The nearly circular mouth. In man}' specimens a 

 part of the convoluted stomach may be found to project out 

 of the mouth. 



d. The ab-oral tentacles: delicate tubular structures 

 which project from the ab-oral wall of the bodj-, among 

 the spines. 



II. Study the manner in which the living animal moves 

 by the use of the ambulacra, 



III. The Digestive Organs. 



Cut off the tip of one of the rays of the tri\ium, and 

 notice the cut ends of the hepatic coeca : two brown sac- 

 culated organs which are attached by a mesenteric mem- 

 brane to the inner surface of the al)-oral wall of the ray. 

 There is one on each side of the median line, and they 



