162 ANARTHROPODA 



specimens are often found with as few as one or two arms 

 full length and the others appearing as mere buds. This 

 is due to the fact that the arms have been broken off and 

 are just beginning to grow. If all five arms are cut off 

 they will be reproduced. The skeleton of the animal con- 



PlO. 187. Dorsal or top surface of the starfish ; m, madreporic plate. Photo- 

 graph half natural size. 



sists of flat, irregular calcareous plates imbedded edge to 

 edge in a leathery skin containing numerous pores through 

 which fingerlike processes project and function as gills. 

 The madreporic plate forms a sieve through which the water 

 passes into the stone canal leading down to the ambulacral 

 or water-vascular system. The latter consists of a vessel 

 encircling the mouth and sending a branch into each arm 

 to connect with the feet. The numerous feet are visible 



