BIVALVE MOLLUSCANS. 



ture tlirough which the fixing tendon passes ; 

 affording an admirable instance of variation 

 in the means of attaining the same end, when 

 circumstances require it. It was necessary 

 that the valves should not be reversed, a tendon 

 through the lower valve secures this in the 

 first of these animals ; but in the second, where 

 the overhanging beak would interfere with 

 this purpose, the tendon issues from the beak 

 itself, so as to enable the animal still to fix 

 itself with the proper valve downwards. In the 

 Anomia the valve takes the form of the sub- 

 stance it is fixed to. 



Who would think that these headless animals, 

 unprovided with organs that indicate any of 

 the higher senses, as sight, smell, and hearing, 

 and apparently fitted with no other means of 

 motion than those of opening and shutting the 

 valves of their shells, or travelling very slowly 

 for a few inches, should yet be able not only to 

 leap and use other motions, but occasionally to 

 sail gaily on the surface of the ocean ; but, 

 however improbable this may seem, it has been 

 proved to be the case by the evidence of eye- 

 witnesses of the fact. 



The common cockle, 1 Poli says, can not only, 

 by means of its foot turn round, or to either 

 side, but even take a good leap. The Trigons, 2 



1 Cardinal edule. ~ Triyonia. 



