SPECIAL PRECAUTIONARY HABITS 331 



become extinct, so that their taking to deep-sea life has proved 

 a protective measure. The nature of these forms and the con- 

 ditions under which they live will be discussed elsewhere. Lakes 

 also possess a deep-water fauna. 



BURROWING MARINE FORMS. As explained elsewhere (p. 249), 

 many marine Bivalve Molluscs provided with siphons are able 

 to feed and breathe when buried in sand or mud, the tips of the 

 siphons only projecting. This is a very effective protective measure, 

 especially as sense-organs are present, which enable their owner to 

 distinguish between degrees of light and darkness. As one result 

 of this, a passing shadow, often an indication of the presence of 

 enemies, causes the siphons to be drawn in. Other bivalves 

 burrow in stone or wood, and the same is true for the members 

 of some other groups. A large number of Marine Worms dwell 

 in mud or sand like their terrestrial brethren, and enjoy a certain 

 amount of protection as a result, though here again certain pre- 

 daceous forms have adopted the same mode of life with intentions 

 the reverse of benevolent. 



