278 THE MECHANICAL FUNCTIONS. 



gular cone. As each ring is so much smaller 

 than the one behind it as to admit of being 

 received within it, the whole head, when com- 

 pletely retracted, presents a flat surface. When 

 this disk is applied to the sand, the animal, by 

 gradually projecting the cone, and successively 

 dilating the rings of which it is composed, opens 

 for itself a passage through the sand, and then 

 secures the sides of the passage from falling in 

 by applying to them a glutinous cement, which 

 exudes from its skin, and which unites the par- 

 ticles of sand into a kind of wall, or coating. 

 This covering does not adhere to the body, but 

 forms a detached coherent tube, within which 

 the animal moves with perfect freedom, and 

 which it leaves behind it as it progressively 

 advances : so that the passage is kept pervious 

 throughout its whole length by means of this 

 lining, which may be compared to the brick- 

 work of the shaft of a mine, or tunnel. 



An apparatus of a more complex description 

 is provided in the Terebellce conchilegce, belong- 

 ing to a tribe of marine worms, which from the 

 peculiar circumstances of their situation, inha- 

 biting parts of the shore nearly midway be- 

 tween high and low water, are obliged often to 

 prolong their tubes to a great length through 

 the sand ; for, in consequence of the frequent 

 shifting of the sands in storms, these animals 

 are sometimes buried to a considerable depth, 



