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 pedicellarize, 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. 
