THE STARFISH AND SOME ALLIES 



225 



muscular. There are valves in the branches of the radial 

 canals which lead to the tube feet. The entire system is 

 filled with water which enters through the sieve plate. It 

 is a well-known fact in physics that water is not readily 

 compressible. This means that when the tubes are filled 



" 



< 



5* " J 



Regenerating 

 ray- 



Anus 



Intestinal cseca- 



Sieve plate 



Pyloric cxcumj 

 cross section 





-rvKtrFosition 

 ^ -of eye 



'Ambulacral 

 plates 



-'—Ampulla 



St Retractor muscle 



of stomach 

 ~~ Stomach pouch 



jjg^— Reproductive gland 



Tube of pyloric cseca 



- %- Pyloric caecum 



Tube foot---- -*>£ -r^yfy 



Reproductive gland 

 cross section 



mm 



IB 



Fig. 122. Dissection of a starfish ; aboral view. (Slightly reduced) 



with water a contraction of the muscles in the ampullae 

 forces the water out into the tube feet. But since the tube 

 feet are already filled with water, their muscles have to re- 

 lax and permit the tubular part of the foot to stretch out 

 longer in order to make room for the water entering under 

 pressure from the ampullae. When a whole group of tube 

 feet are thus extended to full length, the disks on the ends 

 of the feet are pressed against some object and by suction 

 become attached. 



