PART I. 

 A GENERAL HISTORY OF INFUSORIA 



Sect. I.— OF THE BACILLARIA. 



Under this designation, contrived by Ehrenberg, two families of microscopic 

 unicellular Alga) are comprehended, viz. the Desmidie^. and the Diatome^. 



The Diatomea) differ from the Desmidieae chiefly by their dense silicious 

 envelope, composed of two opposite portions or valves and of an interposed 

 segment, and by the general absence of the usual green colouring matter of 

 plants — chlorophyll or chromule. The Desmidieae, on the contraiy, have a 

 non-sihcious envelope, separable into two segments, and filled with bright 

 grass-green chromule. In various vital phenomena the two tribes accord ; 

 but whilst the Desmidieoe are all but universally admitted to be plants, the 

 Diatomese are still regarded by many to be of an animal natm-e. With respect 

 to this question, the arguments ^^ro and con. wiU be best understood when 

 •the organization and vital endowments of these beings have been discussed. 



I.— OF THE FAMILY DESMIDIE^ OR DESMIDIACE^. 

 (Plates I. II. III. and XYI.) 



The Desmidiese are (pseudo-)uniceUular Algae of a herbaceous green colour, 

 of freshwater habit, and have a membranous lorica composed of two symme- 

 trical segments or valves. In Kiitzing's arrangement {8p. Alg.), the Desmidieae 

 constitute a family of the Chamaephyceae, a suborder of the class Isocarpeae. 

 Ehrenberg treated the genus Closterium as a distinct family, which he placed 

 between the Yibrionia and Astasiaea, with the name Closterina. 



That the Desmidieae are actually unicellular (in the sense of forming a 

 single enclosed cavity), Mr. Ralfs has, in his most valuable monograph on the 

 family (1848), taken much pains to demonstrate. 0^^dng to the very deep 

 constriction of the fronds of many genera, e. g. of Euastrum and Micrasterias, 

 the appearance of the little organism is that of two cells united by a narrow 

 band (I. 1, 2, 24, 26, 27; II. 18, 28), forming, in Ehrenberg's opinion, a 

 binary cell or frustule. However, between such deeply partite forms, and 

 others in which no constriction is j)erceptible, for instance in Closterium, 

 eveiy intermediate gradation is met with. Other evidence of the unicellular 

 structure is afforded by the phenomena of conjugation and of the formation 

 of sporangia, by the newly-formed segments resulting from self-fission being 

 interposed between the old valves, and by the fact that the entire contents 

 wiU escape through an opening made in either valve. Moreover, in several 

 genera the circulation of portions of the contents throughout the frond, from 

 one segment to the other, clearly demonstrates the continuity of their interior. 



Figure. — There is great variety in the fig-ure of Desmidieae, and much 



