306 



COLLEGE ZOOLOGY 



rial of unknown function, possibly excretory. 

 The fluid in the coelom is kept in motion 

 by cilia and carries the absorbed food to all 

 parts of the body. 



Excretion 



Excretion is accomplished by the amoebo- 

 cytes in the coelomic fluid, which pass to 

 the outside of the body through the walls 

 of the dermal branchiae. 



Respiration 



Respiration is carried on by means of the 

 dermal branchiae (papulae), which look 

 like a soft furry substance, on the aboral 

 surface of the rays. This appearance is 

 caused by the outpouchings of the thin lin- 

 ing of the body cavity through minute open- 

 ings in the skeleton. These dermal branchiae 

 are covered with cilia on both the inside 

 and outside. The external cilia keep a cur- 



FiGURE 192. A starfish opening a clam. The starfish attaches its tube feet to the two shells 

 of the clam and, by a continuous pull, virtually at right angles to the surface of each shell, 

 eventually opens it. Then the stomach of the starfish is everted through the mouth and brought 

 in contact with the soft parts of the clam (Fig. 190), and the bivalve is actually digested in its 

 own shell. Note that this starfish has lost one ray. (Courtesy of George G. Lower.) 



rent of oxygenated water passing over the 

 branchiae on the outside, and the internal 

 cilia cause the body fluid to flow out into 

 the branchiae. While the body fluid is in 

 the branchiae, an exchange of oxygen and 

 carbon dioxide takes place exactly as it does 

 in our own lungs when the blood flows past 

 the tiny air sacs in them. 



Nervous system and sense organs 



Besides many nerve cells which lie among 

 the epidermal cells, there are ridges of nerv- 



ous tissue, the radial nerve cords (Fig. 

 193), running along the ambulacral grooves, 

 and uniting with an oral nerve ( circumoral ) 

 ring encircling the mouth. In each ray there 

 is (1) a radial nerve cord, (2) a pair of 

 nerves that are aboral to the radial nerve 

 cord, and (3) a nerve cord in the aboral 

 peritoneum. The tube feet are the principal 

 sense organs. They receive fibers from the 

 radial nerves. At the end of each ray is a 

 small, soft, tactile tentacle and a light-sensi- 

 tive eye spot. The dermal branchiae are 

 probably sensory also. 



