270 



THE MICROSCOPE AND ITS REVELATIONS. 



by the straining of repeated quantities, a considerable accumulation may 

 be gradually made. This should be then scraped-off with a knife, and 

 transferred into bottles with fresh water. If what has been brought up 

 by hand be richly charged with these forms, it should be at once deposited 

 in a bottle; this at first seems only to contain foul water, but by allowing 

 it to remain undisturbed for a little time, the Desmids will sink to the 

 bottom, and most of the water may then be poured-off, to be replaced by 

 a fresh supply. If the bottles be freely exposed to solar light, these little 

 plants will flourish, apparently as well as in their native pools; and their 

 various phases of multiplication and reproduction may be observed during 

 successive months or even years. If the pools be too deep for the use of 

 the hand and the scoop, a collecting-bottle attached to a stick ( 216) may 

 be employed in its stead. The ring-net ( 216) may also be advantage- 

 ously employed, especially if it be so constructed as to allow of the ready 

 substitution of one piece of muslin for another. For by using several 

 pieces of previously wetted muslin in succession, a large number of these 

 minute organisms may be separated from the water; the pieces of muslin 



Various phases of development of Pediastrum granulatum. 



may be brought home folded-up in wide-mouthed bottles, either sepa- 

 rately, or several in one, according as the organisms are obtained from 

 one or from several waters; and they are then to be opened out in jars of 

 filtered river-water, and exposed to the light, when the Desmids will 

 detach themselves. 



270. Pediastrece. The members of this family were formerly included 

 in the preceding group; but, though doubtless related to the true Des- 

 midiacece in certain particulars, they present too many points of differ- 

 ence to be properly associated with them. Their chief point of resem- 

 blance consists in the firmness of the outer casing, and in the frequent 

 interruption of its margin either by the protrusion of ' horns' (Fig. 161, 

 A), or by a notching more or less deep (Fig. 162, B); but they differ in 

 these two important particulars, that the cells are not made up of two sym- 

 metrical halves, and that they are always found in aggregation, which is 

 not except in such genera as Scenodesmus (Arthrodesmus, Ehr.), which 



