STRUCTURE OF THE STARFISH. 57 



by a fire, for about twelve hours. The dried specimens 

 should be used to study the hard parts, and the alcoholic 

 or fresh specimens for the internal structure. 

 1. In a dried specimen, notice : — 



a. The central pentagonal disc, from which radiate five 

 arms, or rays. 



b. The nearly flat actinal or oral surface. 



c. The more convex ab-actinal or ab-oral surface. 



d. Upon the oral surface, notice : — 



1. The central pentagonal mouth. 



2. The five clusters of spines, or mouth papillce, which 

 surround and project over the opening. 



3. Five grooves or furrows, the ambulacral furrows, 

 which radiate from the sides of the mouth along the oral 

 surfaces of the rays to their tips. The furrows are deep- 

 est and widest at their central ends, and decrease in size 

 towards the tips of the rays. 



e. ^lake a sketch of the oral surface, showing these 

 points. 



f. On the ab-oral suiiace, notice : — 



1. The integument^ or per isoma, made up of an irregu- 

 lar network of calcareous ossicles carrvnng short blunt 

 spines. The spaces between the ossicles are filled by a 

 soft flexible membrane. Along the middle of the ab-oral 

 surfjice of each ray the spines form an indefinite line. 



2. Near one end of the central pentagonal disc, and 

 opposite an interradius, or point of meeting of two rays, 

 notice a white, circular, raised tubercle, the madreporic 

 body. ^Mien examined with a lens its surface is seen to 

 be marked hy fine undulating radiating lines, which give 

 to it the a})pearance of a piece of madrepore coral. 



3. The ray which joins the disc on the side opposite the 

 madreporic body is the anterior ray. 



