134 THE STARFISH 



other attached forms which are so abundant upon the exposed sur- 

 faces of many marine animals. 



(f) Inject a small amount of India ink or powdered insoluble car- 

 mine into the coelom of a living starfish. Observe the surface carefully 

 with a handlens or low-power of the microscope, or better with a dis- 

 secting binocular microscope. The injected particles can be seen, 

 passing up one side of the cavity within a papula and down the other. 

 The movement of the particles is caused by cilia. The action of cilia 

 on the outer surfaces of the papulae also may be demonstrated by 

 India ink or carmine. These external cilia beat toward the tips of the 

 papulae. How may these internal and external cilia aid in respiration 

 and excretion? 



Exercise 2. — External Features, 



(g) Examine under water a specimen preserved in formalin. A 

 dried specimen will be furnished for comparison as you proceed with 

 the dissection of the formalin specimen. The main divisions of the 

 body are the disk and the arms. On one surface of the disk between 

 two of the arms is a conspicuous plate, the madreporite. Examine 

 its surface with a handlens. The two arms nearest the madreporite 

 are termed the bivium; the other three arms are the trivium. The 

 surface on which the madreporite is found is the aboral surface. In 

 the center of the aboral surface is the anus; it is doubtful whether the 

 anus functions in egestion since it is an extremely small opening. 

 Opposite the aboral surface is the oral surface. The mouth lies in the 

 center of the disk on the oral surface surrounded by a membrane, the 

 peristome. Specimens preserved in formalin may show a soft structure 

 protruding from the mouth. This is part of the stomach, which is 

 regularly everted in the process of feeding. 



(h) Examine the spines on both surfaces of the animal. Are they 

 actually on the surface or do they have a fleshy covering? What is 

 their relation to the underlying skeleton? Note the oral spines around 

 the mouth. What is their function? Examine this region in neighbor- 

 ing specimens. The grooves on the oral surfaces of the arms are the 

 ambulacral grooves (c/. Fig. 63). Do they vary in width in different 

 specimens? How is their width changed? Examine the cylindrical 

 tube feet which protrude from the ambulacral grooves. These are 

 locomotor organs by means of which the animals can crawl in any 

 direction. How can the tube feet be protected from injury? 



(i) Study the aboral surface, using handlens and keeping the 

 specimen under water. Note the size and distribution of the spines 

 and the soft fingerlike papulae which have been previously examined 



