ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



1005 



wticli firmly embrace one another by tbeir sharp edges ; and these two 

 shells can be more or less easily separated by pressure. An exception 

 is presented in Spirotsenia, where the whole cell-membrane consists of a 

 single connected piece, and the author proposes to unite this genus with 

 Mesoteenium and Cylindrocyslis into a distinct group intermediate bet ween 

 the Desmidiege and Zygnemacefe. In some species^ of Penium and 

 Clostermm the cell-wall consists of more than two pieces, and each of 

 the two shells is also provided with a girdle-band formed subsequently. 

 The author considers this structure of the cell-membrane to indicate an 

 evident affinity between the Desmidiese and the Diatomacese. _ 



When the cell is about to divide, a short cylindrical piece of cell- 

 wall is first of all intercalated on the inside of each cell on the line of 

 contact, and this becomes set free by the separation of the two shells. It 

 is only in some species of Clostermm that the wall of the two shells opens 

 by a transverse fissure. A cylindrical cushion is then formed on its inner 

 side, which gradually developes into a complete septum. A fresh half- 

 cell is gradually formed on the inner side of the newly-formed septum. 



Independently of the well-known warts, spines, &c., the cell-wall of 

 desmids is, in almost all cases, perforated by regularly arranged pore- 

 canals, through which pass fine threads of protoplasm from the interior 

 of the cell, ending on the outside in smaller or larger knobs. The warts, 

 spines, and ribs of the cell-wall are hollow, and are usually destitute of 

 these pore canals. 



The majority of desmids are invested in a narrow or broader gela- 

 tinous envelope, which is sometimes easily visible, in other cases only 

 by the use of staining reagents. This envelope is always composed of 

 caps or prisms, placed separately on the pores of the cell-wall, and 

 usually closely connected into a continuous layer. These prisms are in 

 many cases (Jjidymofrium, &c.) i3enetrated by tufts of finer threads pro- 

 ceeding from the knobs which terminate the threads which perforate the 

 pores, ending in very fine cilia at the surface of the envelope. The 

 gelatinous envelope of the individiial cells is at times thrown off, and a 

 fresh envelope then excreted by the cell. In those sj)ecies in which no 

 evident pores were detected in the cell- wall there was also no enveloping 

 gelatinous sheath. The author believes that the substance of the gela- 

 tinous envelope is excreted from the protoplasm of the cell through the 

 pores, and that its chief purpose is to protect the knobs of the threads 

 and especially the tufts of delicate cilia. Whether these serve for the 

 conduction of irritation, or for absorption or excretion, he was unable to 

 determine. 



In those species where the cells are united into filaments, the end- 

 surfaces are also perforated, but exhibit no visible jelly ; except that in 

 Sphserozosma the individual cells are surrounded by jelly, and in Des- 

 midium jelly is also formed in cavities of the septa. The end-surfaces 

 of the cells of these filamentous desmids are either altogether in contact 

 {Hyalotlieca mucosa) or only at certain points (Desmidium, Didymoprium) ; 

 and although threads of protoplasm could not with certainty be detected 

 passing through the pores, it is most probable that the protoplasm of 

 the filament is in this way connected through its whole length. 



In young shells formed as the result of cell-division, the gelatinous 

 envelope is usually not formed until after the shell is fully developed. 

 Up to this time, in the filamentous forms, the growing shells are covered 

 and protected by the envelopes of the old shells. The pores are also 



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