DESMIDACEiE. 413 



seen at the edges. In tlie latter, the matter circulated is in globules, 

 passing each other, in distinct lines, in opposite directions ; in the cir- 

 culation as seen at a, the streams are broad, tortuous, of far greater 

 body, and passing with much less rapidity. To see the centre circu- 

 lation, use a Gillet illuminator and the l-8th power : work the fine 

 adjustment so as to bring the centre of the frond into focus, then 

 almost lose it by raising the objective ; after this, with great care, 

 work the milled head till the dark body of the endochrome is made 

 out ; a hair's-breadth more adjustment gives this circulation with the 

 utmost distinctness, if it is a good specimen. It will be clearly seen, 

 by the same means, at all the points where the spaces are put ; and from 

 them may be traced, with care, down to both extremities. 



The endochrome itself is evidently so constructed as to admit of 

 contraction and expansion in every direction. At times the edges are 

 in semi-lunar curves, leaving uninterrupted clear spaces visible between 

 the green matter and the investing membrane ; at other times the 

 endochrome is seen with a straight margin, but so contracted as to 

 leave a well-defined transparent space along its whole edge, between 

 itself and the exterior case. It is interesting to keep changing the 

 focus, that at one moment we may see the globular circulation be- 

 tween the outer and inner case, and again the mere sluggish move- 

 ment between the inner case and the endochrome. 



At B is given an enlarged sketch of one extremity of a C. Lwnula. 

 The arrows within the chamber pointing to b denote the direction of a 

 very strong current of fluid, which can be detected, and occasionally 

 traced, most distinctly ; it is acted upon by cilia at the edges of the 

 chamber, but its chief force appears to come from some impulse given 

 from the very centre of the endochrome. The fluid is here acting in 

 positive jets, that is, with an almost arterial action ; and according 

 to the strength with which it is acting at the time, the loose floating 

 bodies are propelled to a greater or less distance from the end of 

 the endochrome ; the fluid thus impelled from a centre, and kept in 

 activity by the lateral dlia, causes strong eddies, which give a twist- 

 ing motion to the free bodies. The line — a, in this diagram, denotes 

 the outline of the membrane which encloses the endochrome ; on both 

 sides of this cilia may be detected. The circulation exterior to it 

 passes and repasses it in opposite directions, in three or four distinct 

 courses of globules ; these, when they arrive at — c, seem to encounter 

 the fluid ^e^ec^ through an aperture at the apex of the chamber ; which 

 disperses them so much, that they appear to be driven, for the most 

 part, back again on the precise course by which they had arrived. Some, 



