24 THE ORIGIN OF MAN 



animals, but when the individuals or polyps are 

 minute, their food consists of microscopic ani- 

 mals or plants. Around the mouth is situated 

 radially a number of hollow, finger-like tentacles 

 and much of this tentacular end, or even the en- 

 tire outer surface, is covered with innumerable 

 stinging cells. 



The medusae or jellyfish are very soft, disc or 

 bell-shaped animals with the mouth and stomach 

 hanging from the under side of the bell, while 

 around its edges are the stinging cells. 



The hydrozoa or hydroids (named after the 

 nine-headed dragon of Greek mythology because 

 of the many polyps) usually have chitinous ex- 

 ternal skeletons and a digestive cavity which is 

 undivided and relatively simple in that the walls 

 of the sac are not folded or ridged, while the 

 mouth is rounded. 



The graptolites were linearly arranged colonial 

 hydroids with thin chitinous tubular buds, one 

 end of which was fastened to a rod-like axis. The 



