114 PHYLUM ECHINODERMATA 



In the aboral surface of the disk notice the red madreporite (in 

 preserved specimens it may have lost its color and be white). 

 Examine it on the dried specimen with the aid of a hand lens or 

 the low power of a compound microscope and notice its porous 

 structure. In the aboral surface is also the anus ; it is a very 

 small opening and will be difficult or impossible to see in the 

 specimens at hand. Note the short fixed spines covering the 

 entire aboral surface. Each one is a part of small calcareous 

 plate buried beneath the integument. The entire body wall of 

 the animal is made up largely of these plates, which give it its 

 stiffness. The plates are not, however, connected with one an- 

 other except by muscles and connective tissue, and the animal's 

 arms are flexible and freely movable. Demonstrate this fact 

 with your specimen. In the dried animal this flexibility no 

 longer appears, as the entire body wall has been rendered rigid 

 by the drying. In the soft places between the plates note 

 the delicate, tubular projections of the integument ; they are 

 the contractile papulae, and are organs of respiration and excre- 

 tion and possibly also of sensation. With the aid of a hand lens 

 find, around the base of each spine, the pedicellariae ; these are 

 minute, pincer-like organs of somewhat uncertain function, but 

 they probably aid in keeping the surface of the animal free 

 from particles of dirt and from minute organisms which might 

 be harmful. 



The two arms which inclose the madreporite between their 

 bases are called the bivium; the remaining three, the 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 ambulacra! 

 groove. Observe the two rows of movable spines which fringe 

 each side of the groove ; note 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. 



