THE ORIGIN OF MAN 25 



polyps were arranged in a single linear series 

 (mongraptids) or there were two series back to 

 back against the axis or even four series with 

 the rod in the center. A variable number of 

 such colonies were fastened by rods to a float 

 or swimming-bell from which they hung and in 

 this way they were freely suspended in water and 

 drifted along with sea currents. There was also 

 another group of these animals known as den- 

 droids or net-like graptolites and these were of a 

 very different habit, being attached either to sea- 

 weed or to the sea bottom. These polyps did not 

 have the axial rod but were arranged on flexible, 

 more or less irregular branches united to one an- 

 other by cross-bars. 



The Anthozoa or flower animals include the 

 sea anemones and corals. They are attached 

 polyps whose internal cavity has hanging down in 

 it a rudimentary, tubular, digestive gullet, open- 

 ing upward into an elongated mouth and below 

 into the true digestive cavity. 



