THE THOLAS. 



153 



is thicker and armed with much stronger spines than any 

 other part. After a minute account of the peculiar muscles 

 by which, with the assistance of the foot, the shell is worked, 

 Mr. Osier goes on to say, — " The Pholas, then, has two me- 

 thods of boring. In the first, it fixes itself by the foot (a, 

 Fig. 27), and raises itself almost perpendicularly, thus pressing 

 the operative part of the shell upon the substance to which it 

 adheres. It now proceeds to execute a succession of partial 

 rotatory motions, effected by the alternate contraction of 

 the lateral muscles, employing one valve only, by turning 

 on its side and immediately regaining the erect position. 

 This method is almost exclusively employed by the very 

 young animals, and it certainly is particularly well adapted 

 for penetrating in a direction nearly perpendicular, so that 



Fig. 27. 



they may be completely buried in the shortest possible 

 time ; a time still farther diminished by their form : for at 

 this early age the posterior extremities of the valves are 

 much less produced than they afterwards become. 



" But when the Pholades have exceeded two, or at the 

 utmost, three lines in length, they change the direction and 

 work horizontally ; for the altered figure of the shell, and 

 the increased weight of that part of the animal behind the 

 hinge, prevent them rising so perpendicularly as at first. In 

 the motions required to enlarge the habitation, the adduc- 

 tors perform a very essential part. The animal being 

 attached by the foot, brings the anterior points of the shell 

 into contact. The lateral muscles now contract, and raising 

 the posterior extremity of the shell, press its operative part 

 against the bottom of the hole, and, the moment after, the 

 action of the posterior adductor brings the dorsal margins 

 of the valves into contact; so that the strong rasp-like 



