154 THE TEREDO. — LITIIOniAGA. 



portions are suddenly 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 adductor, 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 per- 

 severingly 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 (Fig. 27, b), 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 particles 

 washed out of it will be deposited before they reach the 

 mouth of the hole, the passage along which the Pholas 

 projects its siphon, is constantly found to be lined with a 

 soft mud. 



The Teredo, according to Mr. Osier, is also a mechanical 

 borer, and it does its work much in the same manner, and 

 by means of a structure very analogous to that of the Pholas. 

 The muscles, indeed, vary in their relative size, because 

 their size is proportioned to the force they are required to 

 exert, which differs in the two genera ; but their arrange- 

 ment and mode of action are so similar, that it is unneces- 

 sary to enter into the detail. The Teredines, it would 

 seem, however, do not eject as useless all the debris worn 

 down in their operations, but turn part of it, at least, to 

 their nourishment, for Mr. Hatchett found the contents of 

 the intestine to be " vegetable sawdust." It is also worth 

 remarking, that they bore in the direction of the grain, 

 whether the wood be erect or otherwise ; and they work across 

 the grain of the wood as seldom as possible ; for after they 

 have penetrated a little way, they turn and continue with 

 the grain tolerably straight until they meet with another 

 shell, or perhaps a knot, which produces a flexure, the 

 course and size of which depend on the nature of the 

 obstruction, and which, if considerable, causes the indivi- 



