204 



ZOOLOGY 





'St ■■.. a I 





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Vertical section through one arm of a starfish; b, ampulla; d, water canal opening at maJre- 

 poric plate {st); i, radial water tube; m, mouth; ft, tube feet; py, digestive gland; stc, 

 stomach. Davison, Zoology. 



On the dorsal (aboral) side of the animal you will find a raised body about 

 the diameter of a pencil. This body, which under the microscope has the 

 structure of a very fine sieve, is called the madreporic plate. Through this 

 plate water passes into a system of water tubes and reservoirs. These tubes 

 extend ultimately into the rays, there ending in the individual tube feet. 



Method of Locomotion. — If we could examine the connection of a 

 tube foot with the system of water canals, we should find that water pass- 

 ing from the canals in the rays flows into a tiny receptacle connected with 

 the tube foot. If the tube foot is to be extended, the muscles in the wall 

 of the ampulla (the receptacle next the tube foot) contract. This forces the 

 water out, closing a tiny valve on the side of the radial canal and sending 

 the water into the tube foot, thus causing it to lengthen. The end of the 

 tube foot is composed of a little disk of muscle, thicker at the outer side 

 than on the inner. When this disk is placed against an object, and when 

 water is withdrawn from the tube foot into the ampulla, the disk becomes 

 fastened again by suction. If now the muscles in the wall of the tube 

 foot contract, and this process takes place simultaneously and in hundreds 



of the feet, it can be seen that the body of the 

 starfish is drawn forward a short distance. To 

 release the tube foot water is pumped in from 

 the ampulla by the process mentioned above. 

 The same act repeated again and again results 

 in locomotion at the rate of about six inches per 

 minute in an adult starfish. 



The Nervous System and its Work. — The 

 movements of the tube feet, although not in 

 unison, are coordinated to act for a common 

 purpose. If the animal starts to move in a given 

 direction, the tube feet in the different rays all 

 pull in the same direction. It has been found 

 that this is due to the presence of a ring of 

 nervous tissue, strands of which extend out into 



Diagram of nervcus system of 

 s t a r fi s h ; r, nervous ring 

 around mouth; n, radial 

 nerves to each arm, ending 

 at the eye. 



