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 TereheUce 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, 



