COMMENSALISM AND PARASITISM. 163 



Many of the Worms use little bits of shell and 

 grains of sand to build up a tube for the pro- 

 tection of their bodies. One of these the Tere- 

 bella is very common on our shores, the sandy 

 tubes ending in a tuft of fine filaments, and 

 decorated all over with tiny little stones or 

 shells, projecting an inch or two from the sur- 

 face of the sand. In some localities these tubes 

 may be found in thousands when the tide is 

 low. 



Another form Pectinaria constructs much 

 firmer tubes, which retain their cylindrical shape 

 after the death of the animal. In the process of 

 construction this Worm must carefully select the 

 grains of sand, for when the tube is examined 

 with a magnifying glass the particles will be 

 seen to be of almost exactly the same size, and 

 arranged in their places with a mathematical 

 precision. 



But Worms are not by any means the only 

 animals that use the sand in this manner for the 

 protection of their bodies. There are some 

 kinds of Polyps, belonging to the family Zoan- 

 thidce, a peculiar group of Sea-anemones, in 

 which the body-wall is considerably strengthened 

 by foreign bodies of various kinds. The Zoan- 

 thus does not, like the Terebella-worm, form a 

 tube or case in which the body can freely move 

 up or down, but sticks the grains of sand into 

 its skin, so that they become in the older forms 

 deeply buried in the tissues and give a consider- 

 able support to the body-wall. 



The Cerianthus another Sea-anemone forms 

 a tube which is partly composed of a matted 



