254 THE BORING OPERATION 



Then follows an expansion of the neck or upper part 

 of the ventral border, from whence the siphons pro- 

 trude. This movement closes the posterior portions 

 of the valves below the hinge, and brings their 

 serrated points together. The next act on the part 

 of the animal is to place its foot firmly at the base 

 of the hole ; when leaning forward, it makes a 

 sweeping movement fully half round the cavity, 

 pressing firmly upon the umboes, which nature has 

 strengthened for the purpose by two curved teeth 

 fixed on the inside of the valves. At this stage it 

 again reclines on its breast, and tilting up the shell 

 as much as possible, it makes another motion round 

 to its former position, leaning upon its back. By 

 these intricate movements, which the Pholas appears 

 to accomplish by a contraction almost painfully 

 strong, it opens the rasping points of the valves. 

 These execute a very peculiar scooping movement at 

 the base of the cavity, and the animal having got so 

 far, prepares itself for further exertion by a short 

 rest. 



The specimen whose movements I have attempted 

 to describe, lived in my possession for a considerable 

 time. It bored so completely through the piece of 

 rock in which it was embedded, that the whole of 

 its foot dropped through the aperture, and remained 

 in this position for months, the animal, in conse- 

 quence, being unable to change its position even in 

 the slightest degree. Each movement of this speci- 



