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Burrowing and Stationary Mollusca. 257 



first, it fixes itself by the foot, and raises itself almost perpen- 

 dicularly, 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 alter- 

 nate 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 perpen- 

 dicular, so that 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 adductors per- 

 form 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 portions are sud- 

 denly separated, and scrape rapidly and forcibly over the 

 substance on which they press. As soon as this is effected, 

 the posterior extremity sinks, and the stroke is immediately 

 repeated by the successive contractions of the anterior ad- 

 ductor, the lateral, and the posterior adductor muscles. 



Thus do these creatures mine their cells; the instinct 

 which directs them operating from their earliest infancy: for 

 they are found completely buried, when so minute as to be 

 almost invisible ; and the rapidity of their growth, for the first 

 few weeks, compels them to exert themselves perseveringly in 

 effecting the enlargement of their habitation. The particles 

 of clay or wood worn down by their operations, and which, in 

 a short time, completely clog the shell, are removed in a very 

 simple manner. The animal fills the siphonal tubes, which 

 convey water into its body, closes the orifices and retracts 

 them suddenly ; by which act the water which they contained 

 is ejected forcibly from the opening in the mantle ; and the 

 jet is prolonged by the gradual closure of the valves, expelling 

 the water contained within the shell. The chamber occupied 

 by the animal is thus completely cleansed ; but as many of the 



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