356 Natural Histoty of Molluscous Aiiimals : — 



structure which enable the animal to perforin the necessary 

 motions, but much of it I must omit. The fish, he tells us, is 

 provided with four sets of muscles, in addition to those belong- 

 ing to the siphon. The posterior adductor (Jig. 62. c) is a 

 flat muscle, connecting the valves nearly midway between the 

 hinge and the extremity of the shell, and having its attach- 

 ments at about one eighth of an inch within its dorsal margins. 

 It lies so superficially, that its action is seen when the animal 

 employs it in boring. The anterior adductor (Jig. 62. d) is 

 attached to the reflected folds of semi-perlaceous shelly matter 

 which cover the umbo. It extends, from a point a little before 

 the hinge, nearly to the anterior extremity of the shell, and 

 is covered by the accessory valve. The ventral margins are 



connected by muscular 

 fibres from the opening 

 in the mantle through 

 which the foot is pro- 

 jected, as far as the 

 origin of the siphon. 

 " A pair of muscles, 



ducelfoSspfayr'"^^ 6, A piece of dark paper intro- ^^Jch may be termed 



lateral, arise from the 

 points of the long hooked processes, which, becoming fan- 

 shaped as they pass over the body, are inserted into the sides 

 of the foot (Jg. 63. a). 



The offices of these muscles are peculiar. The shell is 

 closed, not by the adductors, but by the fibres which connect 

 the ventral margins of the valves, and it is opened by that 

 part of the anterior adductor which lies nearest to the hinge, 

 and which thus performs an office analogous to that of the 

 ligament in other bivalves. The other portion of this muscle 

 antagonises the posterior adductor. By its contraction, the 

 anterior points of the valves are brought into contact, and their 

 dorsal margins separated as widely as possible. The action 

 of the posterior adductor reverses this state ; and, in uniting 

 the dorsal margins, expands the anterior and armed portion 

 of the shell. The foot of the Pholas, like that of the Gas- 

 teropoda, is a flat disc, by which the animal can attach itself 

 firmly. When it is thus fixed, the lateral muscles, acting in 

 an oblique direction, will raise the posterior end of the shelly 

 and press its armed extremity forward and downward ; or, if 

 one of them should contract more strongly than the other, it 

 will bring down the corresponding side of the shell, which 

 will be restored to its erect position by the action of the oppo- 

 site muscle. 



The Pholas, then, has two methods of boring. In the 



