FOSSIL SEA-MATS. 



211 



numbers of the pretty corals, shells, etc., in the lime- 

 stones of the Silurian, Devonian, Carboniferous, 

 Permian, Oolite, Chalk, and Crag formations are 

 more or less covered with fossil Polyzoa. Indeed, in 

 the Crag beds we often find univalves so thickly 

 encrusted with growths of these animals that the 

 mollusca must have been finally killed by the mouths 

 of their shells being closed. Recent fronds of the 

 Laminaria seaweed just thrown up are sure to be 



Fig. 167. Recent Polyzoon, showing Polyps protruded (magnified). 



found with living patches of these lace-like organisms 

 upon them. Let the observer snip off a strip with 

 such a colony upon it, and place it in sea- water. If it 

 is of a dull glassy appearance, and not an opaque 

 white, the colony is probably alive. When the strip is 

 placed in a zoophyte-trough, after a short time, the 

 observer will see suddenly popping out of each cell a 

 cluster of lily-like petals, sixteen or more in number. 



