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. 
