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EVOLUTION OF ANIMALS 



Part V 



Cut end 

 Radial canal 



Bulb of 



lube foot 



(inside body wall) 



Tube foot 

 (outside body wqII) 



Fig. 32.5. Diagram of a part of the water-vascular (circulatory) system of a 

 starfish; three of the radial canals are cut near the base. This system takes part 

 in all movements. It is to the starfish what the circulation of blood is to the 

 human body and more — a waterway constantly receiving water through the 

 sieve plate and constantly expending water carrying other substances with it. 

 Water takes part in every movement of the starfish and in every phase of its 

 living. Tiedemann vesicles are not shown. 



clothed with cilia, whose rapid whipping keeps currents of water moving over 

 the surface. Some of the pincers work like forceps, others like scissors, but all 

 are traps that pinch and hold until they are stimulated by some other contact 

 such as the touch of a neighboring pincer or a falling particle. Multitudes of 

 minute skin gills which freely open from the body cavity are filled with the 

 coelomic fluid that oozes slowly about in any open place (Fig. 32.3). 



Locomotion, Circulation of the Blood, and the Water Vascular System. 

 Starfishes move by manipulating the fluid in the versatile water vascular sys- 

 tem. This contains the circulating fluid that, although largely sea water, may 

 still be called blood since it contains cells and is concerned with respiration. 

 The structures that belong especially to this system are the sieve plate, many 

 canals, the tube feet, and the skin gills. The ciliated stone canal leads from 

 the sieve plate to the circular canal around the mouth. Opening into the latter 

 are nine small sacs (Tiedemann vesicles) in which the ameboid blood cefls 

 originate. Also opening from the circular canal are five radial canals, one to 

 each arm (Fig. 32.5). These connect with each tube foot by a short canal. 

 All the tubes are passageways for the water that enters through the sieve plate 

 and, picking up various substances in the body, becomes the blood. 



The tube feet are so coordinated through the central nervous system that 



