98 FRESH-WATER ALG^. 



brown and invested with a thick cell-wall. The sporangium is 

 terminal, and gives rise to one large zoospore, densely clad with 

 vibratile cilia. Probably reproduction also takes place by means of 

 zoogonidia, as Mr. Bates, of Leicester, found some filaments of 

 Vaiicheria beneath ice in a pool, in which zoogonidia were appar- 

 ently in the course of formation. Some species of Vaiicheria are 

 infested by Cyclops lupula^ which occasions the growth on the fila- 

 ments of extraordinary looking appendages, in the midst of which 

 the parasites reside. 



The Volvox family contains two most pleasing and well known 

 genera, Volvox and Eudorina, the coenobium of the former con- 

 taining a great number of cells, and of the latter either i6 or 32. 

 The genus Eudorina very much resembles Pandoriiia in appear- 

 ance ; there is, however, a great difference between them physio- 

 logically — Pandorina bemg reproduced by conjugation of 

 zoogonidia, as has already been described ; Eudorina by means of 

 antheridia and oospheres. Non-sexual reproduction is efi"ected 

 in Volvox by the repeated division of certain cells — in Eudorina 

 by division of any of the cells. 



■ The well-known rotary movement of Volvox is produced by 

 the combined action of the numerous pairs of cilia in which the 

 gonidia terminate and which protrude through the cell wall. The 

 rapid appearance or disappearance of large numbers of Volvox in 

 the same pool is doubtless due to the fact that a slight change in 

 external conditions suffices, on the one hand, to favour the 

 development of countless thousands of young plants, and, on the 

 other, to destroy the vitality of the colony, or to drive it to seek 

 refuge in deeper water. 



The monograph of Mr. Wills on this genus, to be found in the 

 Midland Naturalist^ for 1880, is a classical one in English botani- 

 cal literature. 



Eudorina is much smaller than Volvox, and also possesses the 

 same rotary motion. It also appears and disappears in the same 

 rapid manner as Volvox. 



The family yEdogoniece contains two genera, yEdogoiiium and 

 Bulbochoite, which are fixed at the lower end often to the sub- 

 merged parts of other plants. 



In yEdogo?iiu?n the thallus is unbranched, in Bulbochate it is 



