TEREBRATING POWERS OF THE PHOLAS. 259 



siphons to the eyes of a greedy crab, it is perfectly 

 safe from attack. It is only when the fleshy foot 

 is unprotected that it falls a prey to some hungry 

 crustacean. 



The toughness of the siphonal orifices is, I believe, 

 a most important point, for, as I shall endeavour to 

 explain, the siphonal tubes constitute important 

 accessory excavating agents, to those already enu- 

 merated. 



We all know that the hole which each young 

 Pholas makes, when first he takes possession of his 

 rocky home, is extremely minute, not larger than 

 a small pin's head ; now, it stands to reason, that if 

 the shell was the only terebrating agent, the open- 

 ing of the cavity in question would always remain 

 of the same size, or, perhaps, on account of the 

 action of the water, a slight degree larger than its 

 original dimensions. Such, however, is not the 

 case. 



Here is a fragment of rock exhibiting several 

 Pholas holes. The aperture of one of these, which I 

 measure, is nearly half-an-inch in diameter, while in 

 juxtaposition with it is situated another cavity, 

 measuring across the entrance less than the eighth 

 part of an inch. The reader will at once perceive, 

 if the foot and shell were the sole augurs, that as 

 the animal descended deeper into the rock, the 

 siphonal tube, as it enlarged in proportion to other 

 parts of the animal, would have to be drawn out to 



