HABITS OF MARINE ANIMALS 157 



for besides the animals that crawl about, every rock, 

 stone and weed is covered with hydroids, sponges and 

 barnacles, and often the rocks themselves are riddled 

 by boring creatures. Even the empty tin or broken 

 bottle thrown over the side of the boat, is soon covered 

 both inside and outside with various forms of marine 

 animals. As an example, look at the illustration of the 

 portion of a broken bottle dredged up outside Plymouth. 

 To it are attached bunches of feathery hydroids, various 

 shell-fish, sponges and masses of ova. Had the hydroids 

 and ova been undisturbed, from the life on this bottle 

 alone many thousands of individuals would have been 

 added to the plankton of the sea. 



