A STARFISH 143 



trivium. How can a plane be passed through the body so as to 

 divide it into two symmetrical halves ? 



Exercise 1. Make a life-size drawing of the aboral aspect of the 

 animal and label all the features observed. 



On the oral surface observe the deep groove which extends 

 from the mouth along each arm to its tip. This is the ambulacral 

 groove. Observe the two rows of movable spines which fringe 

 each side of the groove ; also the five pairs of movable spines 

 which surround the mouth. Separate these spines and observe 

 the mouth surrounded by a circular membrane, the peristome. 



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 discs 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 disc 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 of these features. 



Scrape off several pedicellarise, mount them on a slide, and 

 examine 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. 



