THE ORIGIN OF MAN 23 



superficial radial symmetry, but as they have the 

 true body cavity they are therefore coelomates. 

 Echinodermata are exclusively marine and are of 

 great diversity of external form. 



The phylum Cnidaria includes two widely dif- 

 ferent kinds of individuals: the attached polyps 

 and the free individuals or medusae, which are 

 mostly known as swimming-bells or jellyfish. By 

 the way, Cnidaria comes from the Greek for net- 

 tle, because the majority of these animals have the 

 power of stinging. This stinging is done by a 

 multitude of very minute threads, each one of 

 which is shot out of a cell buried in the skin and 

 enters any soft body that comes in contact with 

 the animals. The body of these animals is a sac 

 with the closed end attached and the other free, 

 the widely distensible opening being the mouth. 

 Into it is passed the food, which is digested in a 

 more or less simply constructed cavity of the sac, 

 and through the same opening any indigestible 

 remainder is ejected. They feed on the smaller 



