244 
POPULAR SCIENCE REVIEW. 
game matter which these organisms take up and deposit in 
their tissues, is “by no means dependent upon the amount 
present in the medium in which they live, but upon the selec- 
tive working of their tissues.”* 
The mechanism of the hinge is very interesting. It consists 
of — 1. The “ligament,” a thickened modification of the peri- 
ostracum, which is attached on either side to a ridge on the 
dorsal margin of the shell-valve, just posterior to the umbo, 
and is, consequently, being elastic, put on the stretch by the 
closing of the valves by the adductor muscle. 2. The “ car- 
tilage,” spring, or internal ligament (c, fig. 1), is lodged on 
either side in a kind of furrow, bounded by two ridges, the 
outer of which gives attachment to the ligament. The inner 
ridge is indicated in fig. 1 by the letter h. This spring is 
composed principally of elastic fibres, having a direction per- 
pendicular to the plane of the surface of attachment, and which 
are, consequently, compressed on the closing of the shell-valve. 
Here we have a good instance of the economy of physio- 
logical labour ; for while the opening of the shell, a condition 
favourable for the respiration and nutrition of the contained 
animal, is brought about by the purely physical means of an 
elastic tissue in a state, so to speak, of equilibrium, the closing 
of the valves, which is but an occasional act (rendered necessary 
by the presence of danger, &c.), is effected by muscular action, 
which involves the outlay of a certain amount of nerve force. 
If this state of things were reversed, the animal would 
become speedily “ tired,” and the “ waste of tissue ” would be 
so much increased as to become quite prodigal. 
Those acquainted with human physiology will, doubtless, 
call to mind the antagonism to the external intercostal muscles 
of the stretched rib-cartilages, and the contained lung tissue, in 
the mechanism of respiration. 
Muscular System . — This is comparatively simple ; com- 
prising only the muscles of the shell and of the foot. 
1. Muscles of the shell. These consist of two cylindrical 
bundles of fibres, which pass transversely from one valve to 
the other at points near their anterior and posterior extre- 
mities, and much nearer to their dorsal than to their ventral 
margins (fig. 1, a a and p a .) By their active contraction, 
awakened by the stimulus of nerve force, they overcome the 
passive antagonism of the ligament and cartilage of the hinge, 
and are termed respectively, from their action upon the shell- 
valves, anterior and posterior “ adductors.” 
Some bivalves, e.g. the oyster, scallop ( Pecten ), and clam 
( Tridacna ), have but one adductor, and were consequently 
* Foims of Avxmal Life. Oxford: 1870. 
