162 THE MECHANICAL FUNCTIONS. 



notices the singular fact that oysters, which are attached to 

 rocks occasionally left dry by the retreat of the tide, always 

 retain within their shells a quantity of water sufficient for 

 respiration, and that they keep the valves closed till the re- 

 turn of the tide: whereas those oysters which are taken from 

 greater depths, where the w^ater never leaves them, and are 

 afterwards removed to situations where they are exposed to 

 these vicissitudes, of which they have had no previous ex- 

 perience, improvidently open their shells after the sea has 

 left them, and by allowing the water to escape, soon perish.* 

 Many bivalve mollusca are provided with an instrument 

 shaped like a leg and foot, which they employ extensively 



for progressive motion. Its form 

 in the Cardium, or cockle, is seen 

 in Fig. ] 04. This organ is com- 

 posed of a mass of muscular fibres, 

 interwoven together in a very com- 

 plex manner, and which may be 

 compared to the muscular structure 

 of the human tongue: the effect in both is the same, namely, 

 the conferring a power of motion in all possible ways; thus 

 it may be readily protruded, retracted, or inflected at every 

 point. The Solen, or razor-shell fish, has a foot of a cylin- 

 drical shape, tapering at the end, and much more resembling 

 in its form a tongue than a foot. In some bivalves the dila- 

 tation of the foot is effected by a curious hydraulic mechan- 

 ism: the interior of the organ is formed of a spongy texture, 

 capable of receiving a considerable quantity of water, which 

 the animal has the powder of injecting into it, and of thus in- 

 creasins; its dimensions. 



The foot of the Mytihis edid'is, or common muscle, can 

 be advanced to the distance of two inches from the shell, 

 and applied to any fixed body within that range. By at- 



A A, which retain the valves in contact independently of the ligaments. For 

 a full description of this apparatus, I must refer to a paper by Mr. Osier, on 

 burrowing and boring marine animals, contained \n the Phil. Trans, for 1826, 

 p. 342, from which tlie above figure has been taken. 

 * Journal do Physique, xxviii. 244. 



