18 Mr. H. J. Carter on the Circulation of the 



again, may be conceived, from what I have stated, to be very 

 rare occurrences, and, as such, are only worth remembering. 

 The only change in the green disk which is frequently observed, 

 is its transition from its common elliptical to a circular form, 

 which generally accompanies a cessation of the circulation ; but 

 not always, for the former is sometimes seen when the circula- 

 tion has been permanently arrested, and the latter when it is 

 going on with great activity. 



The contraction, however, of the green layer as a whole, when 

 the internode sustains even a slight injury, is worthy of notice. 

 The same thing takes place in Zygnema {nitidum, mihi). On 

 one occasion, when testing the evacuated contents of a truncated 

 internode with tincture of iodine, I observed a fragment of the 

 green layer, which was projecting from the orifice of the inter- 

 node, to tremble rapidly when the iodine touched itj this 

 motion, after a few seconds, became rhythmical, and then slower 

 and slower, until it altogether ceased ; thus exhibiting the same 

 kind of convulsive motions as muscular fibre under similar 

 circumstances. When one end of an internode is truncated 

 with a sharp instrument, as before stated, the whole of the loose 

 contents rush out, while the green layer remains within the 

 internode, but retracted both from the orifice and sides of the 

 cell- wall; and it is this contraction which appears to be the 

 principal agency in causing the rapid expulsion of the mucus- 

 layer, which I may here mention is every now and then drawn 

 back into the internode spasmodically, from a contraction, ap- 

 parently, of its own substance, and perhaps a momentary relaxa- 

 tion of the green layer at the same time. In what part of the 

 green layer this contractile power resides, I am not able to 

 state ; but as the green disks are appended to vesicles which 

 thus make up the green layer, and the irregular bodies of the 

 mucus layer, which appear to be almost identical with them, 

 have also an appended vesicle, which, a certain time after they 

 have been exposed to the water, contracts to such an extent as 

 to become undistinguishable from the irregular body itself, it 

 may be the contraction of the vesicles of the green disks which 

 produces the general contraction of the green layer. 



Be this as it may, the movement which is the chief object of 

 our consideration here is that of the mucus-layer. It has already 

 been shown, that this is independent of the green layer; and 

 this, combined with the power of polymorphism and locomotive 

 agency which it presents when investing the groups of gi-een 

 disks pi-cparatory to passing into the firm gonidial cell-wall, 

 seems conclusive of its possessing an inherent power of mobility 

 independent of any other influence, so long as it remains un- 

 altered, and within an uninjured internode. The movement of 



