COMMENSALISM AND PARASITISM. 153 



Living in a tube on our coasts is a very 

 common Worm called Sabella, and at the mouth 

 of the tube a little Polyp may frequently be 

 found which has received the fanciful name of 

 the " Household god of the Sabellids" (Lar 

 Sabellarum).' The Polyp undoubtedly benefits 

 by the currents of water which the Worm sets 

 up when feeding, but it is difficult to see what 

 advantage, if any, the worm 

 gains from the presence of the 

 Polyp. 



Again, some of the Trepangs 

 are frequently inhabited by a 

 little Fish called Fierasfer, which 

 comes out from time to time to 

 feed and " take the air," but 

 rapidly retreats into the body 

 of the Trepang on the slightest 

 alarm. 



The large stinging Sea-ane- 

 mones of the Coral often afford 

 protection of a similar kind to 

 a little Fish. Saville Kent gives 

 a beautiful picture of a little FlG - 

 bright red Fish swimming about 

 on the disc of a large purple Sea-anemone, 

 and he says that it darts into the mouth when 

 alarmed. On our own coasts we may often 

 observe a number of little Fish generally belong- 

 ing to the Cod-family swimming round the disc 

 and tentacles of the large Jelly-fishes, and these, 

 when frightened, swim vigorously toward the 

 under surface of the umbrella and seek security 

 there. Sometimes as many as a hundred or 



