90 THE PHYSIOLOGY OF THE COMMON CRAYFISH. 
Whatever else it may be, this animal, so far as it is 
acted upon by bodies around it and reacts on them, is a 
piece of mechanism, the internal works of which give rise 
to certain movements when it is affected by particular 
external conditions ; and they do this in virtue of their 
physical properties and connexions. 
Every movement of the body, or of any organ of the 
body, is an effect of one and the same cause, namely, 
muscular contraction. Whether the crayfish swims or 
walks, or moves its antenne, or seizes its prey, the imme- 
diate cause of the movements of the parts which bring 
about, or constitute, these bodily motions is to be sought 
in a change which takes place in the flesh, or muscle, 
which is attached to them. The change of place which 
constitutes any movement is an effect of a previous 
change in the disposition of the molecules of one 
or more muscles; while the direction of that move- 
ment depends on the connexions of the parts of the 
skeleton with one another, and of the muscles with 
them. 
The muscle of the crayfish is a dense, white substance ; 
and if a small portion of it is subjected to examination it 
will be found to be very easily broken up into more 
or less parallel bundles of fine fibres. Each of these 
fibres is generally found to be ensheathed in a fine trans- 
parent membrane, which is called the sarcolemma, within 
which is contained the proper substance of the muscle. 
When quite fresh and living, this substance is soft and 
