TUBE DWELLINGS. 195 



and furnishing them the requisite amount of air they 

 have the power of extracting from the current and car- 

 rying the water down into the mouth, whicli opens at the 

 bottom of the gill tufts. In the water is always a suffi- 

 cient supply of minute marine animals to furnish the , 

 serpula, as this worm is called, with the food it requires. 



The tube it inhabits, like the tube of the trapdoor 

 spider, has a lid. This is called an operculum, from 

 a Latin word meaning a cover or lid. 



The serpula, which gets its name from the serpent- 

 like coils ia which its tube is constructed, seems to be 

 built on the reverse principle to that of a Jack-in-the- 

 box, which is always ready to startle the spectator 

 with his sudden appearance, but has to be forced back 

 into his box, while the serpula comes out of its tube 

 very slowly and cautiously, but on the slightest alarm 

 vanishes with lightninglike rapidity back into it, clap- 

 ping its door shut after it. As the tube is from six to 

 ten times the length of the animal that made it, the 

 serpula has a deep and safe retreat. 



It withdraws itself into its shell by means of an 

 array of hooks on the upper part of the body, extend- 

 ing half across the back, that catch into the membrane 

 that lines the tube. These hooks are fastened to long 

 threadlike sinews, by means of which they can be 

 unfastened as well as fastened to the interior surface 

 of the tube. 



There are a great number of species and varieties 

 of these beautiful marine worms, some of which con- 

 trive to form of mud and a secretion from their bodies 

 tubes almost as tough and elastic as India rubber. 



