16 INTRODUCTION. 



and Penium Brebissonii, under such circumstances, form a 

 distinct mucous stratum ; and on this account some authors 

 have placed the latter with the Palmellese, although when it 

 occurs in water it is less gelatinous than many other species 

 belonging to the family of the Desmidiese. 



I have never obtained a clear view of the circulation wit- 

 nessed by various authors in species of Closterium and Doci- 

 dium, but I have no doubt that it is correctly described in 

 the following account by Mr. Dalrymple of his own obser- 

 vations : — " A circulation of the fluids within the shell was 

 observed independent of the active molecules ; this was 

 regular, passing in two opposite currents, one along the side 

 of the shell, and the other along the periphery of the gela- 

 tinous body*." Professor Bailey observes, that "The account 

 by Mr. Dalrymple agrees with what I have witnessed in 

 several species. The currents are very distinct ; so much so 

 in fact, that they attracted my attention before I was aware 

 that they had been noticed by others f." 



Dr. Williams of Swansea has kindly translated from the 

 German a paper on this subject by Labarzewski|, but it is 

 too long for insertion here, nor would it be intelligible with- 

 out the accompanying figures. Labarzewski's observations 

 were made on specimens of Closterium Lunula. The circu- 

 lating fluid was clear and thick, and filled the space between 

 the covering and the central mass of green granular ma- 

 terial, from which granules detached themselves from time 

 to time, and after moving along the margins, returned to 

 their former situation. The current was quickened at the 

 ends and near the suture, where it was lost, but reappeared 

 in the other segment. The circulation was intermittent, 

 lasting each time about seven seconds. 



I now approach a question on which I feel the greatest 

 anxiety, lest I should not do justice to the arguments of 

 those from whose opinion I may diflfer, or should fail satis- 



* Annals of Natural History, vol. v. p. 416. 



t American Journal of Science and Arts, vol. xli. p. 300. 



: Linn^a, 1840. p. 278. 



