A STARFISH 115 



From the sides of each ambulacral groove two zigzag rows of 

 soft tentacles project. These are the ambulacral feet ; they are 

 muscular tubes with sucker disks at their ends and are the organs 

 of locomotion. Scrape the feet from a portion of the groove and 

 examine its sides ; note the slender, transverse calcareous plates 

 which form it, and the round openings between them, called the 

 ambulacral pores, through which branches from the feet project 

 into the body cavity. Note the zigzag nature of each of the two 

 rows of these pores. Notice also the delicate cord which extends 

 along the median line of the groove ; it is the main nerve of the 

 arm ; it proceeds from a nerve ring in the central disk to the tip 

 of the arm. Follow it to the tip and note the red pigment spot 

 with which it ends. This is the eye. In preserved specimens the 

 pigment may have lost its color. 



Exercise 2. Make a life-size drawing of the oral aspect of the animal 

 and label all these features. 



Scrape off several pedicellariae, mount them on a slide, and ex- 

 amine them under a compound microscope. By pressing on the 

 cover glass with a needle, the jaws can be made to open and shut ; 

 try it. 



Exercise 3. Draw a pedicellaria on a large scale. 



Cut off the aboral wall of the severed arm of the dried specimen 

 and scrape away the remains of the internal organs and the am- 

 bulacral feet. Study the inner surface of the ambulacral groove. 

 Note the two rows of slender transverse plates which form the 

 sides of the groove, and on each side between every two plates, 

 the minute ambulacral pore. 



Exercise 4. Make a drawing on a scale of 3 of the inner surface of the 

 ambulacral groove, showing the plates and the pores. 



Cut off the aboral wall of the central disk of the dried specimen, 

 scrape away the remains of the internal organs, and study the ar- 

 rangement of the plates in the inner surface of the oral body wall. 

 Note the circular mouth protected by converging spines, and also 

 the membranous peristome. Observe the convergence of the five 



