1873.] 121 [Dwight. 



views in the range of microscopy; the muscular fibres of apparently- 

 semi-fluid consistency are in almost constant, though irregular, mo- 

 . tion, carrying the smaller branches of the tracheae to and fro, as 

 seaweed is swung by a wave. These phenomena sometimes continue 

 for an hour and a half, with no other encouragement than an occa- 

 sional slight tap on the covering glass. The following observations 

 have been made on the trochanter, the femur and the tibia. In the 

 two former it is easier to find places where there is but one layer of 

 fibres, but the tibia is usually preferable, for. the shell is more trans- 

 parent, and there are many fibres attached at different degrees of 

 obliquity to a tendon, so as to illustrate the effect of more or less ten- 

 sion. Moreover, these fibres, owing perhaps to the proximity of a 

 large tracheal dilatation, preserve their irritability much longer than 

 others. 



The work of which this paper contains the results, has been done 

 entirely on preparations of this kind of the legs of the Gyrinus. 

 The observations were made while the fibres were still capable of 

 contraction. Hartnack's immersion objectives, Nos. 9 and 10 (usually 

 the latter) were employed, with his No. 3 ocular, and the draw-tube 

 drawn out. 



As is well known, the muscular fibres of insects are not bound to 

 gether by interstitial tissue nearly so strongly as those of vertebrates, 

 and each is able to contract freely independently of its neighbors. 

 The states in which muscular fibre is seen may be divided into four 

 chief ones. 



The first state (Plate n, fig. 1 and fig. 2, A) is the essentially nor- 

 mal one in which the fibre is at rest, but extended between points 

 far enough apart, and sufficiently fixed, to oppose a certain resistance 

 to its elasticity and contractility . 



The second state (fig. 3) is seen when the fibre is free from all strain 

 or resistance; it is that which a fibre in the first state would assume 

 if one of its attachments were divided, or moved much nearer to the 

 other. 



The third state (fig. 2, C), usually owing to contraction of another 

 part, is one in which a part of a fibre is put upon a decided stretch. 



The fourth state (fig. 2, B) comprises all the degrees of active con- 

 traction. 



In the^rs^ state the borders of the fibre are straight, and are con- 

 nected by narrow transverse bands of a black granular structure, which 

 present considerable variation. Usually the minute black granules lie 



