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 antennae, 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 



