MioractiniesB 



199 



variously disposed about the periphery of the cells. They are unicellular 



or they consist of small aggregates of cells, 



usually devoid of a mucous envelope. The 



bristles vary in number, length and attachment, 



but in most forms they are considerably longer 



than the diameter of the cells. They are in all 



instances rather delicate and easily overlooked. 



In addition to the more obvious bristles there 



are a number of others of a much more delicate 



character, similar to those which are known to 



occur on Pediastrum and Scenedesmus. (See 



pages 202 and 219, and figs. 130 .F and 144 H.) 



Multiplication rarely occurs by simple 

 division, and the usual method of reproduction 

 is by autospores, formed 2, 4, 8 or 16 in a 

 mother-cell, each autospore often acquiring the 

 full characters of the adult cell before its 

 liberation from the distended mother-cell-wall. 

 It is incorrect to regard the small colonies of 

 these Algas as coenobia, since the number of 

 cells is not definite, not even in those forms of 

 Micractinium described as ' Ricliteriella,' and 

 the aggregation of the cells is a loose one. 



It has been generally supposed that most of 

 the members of this characteristic group 

 occur principally in the plankton of large 

 lakes, but they certainly occur far more abun- 

 dantly in small reservoirs, in ponds, and in 



canals than they ever do in lakes. Chodat ('94) described the occurrence 

 of zoogonidia in Micractinium (= Golenkinia Chodat), but this is open to 

 considerable doubt and may be an error of observation due to contaminated 

 cultures, as in the case of Eremosphxra. No trace of zoogonidia has ever 

 been observed by other investigators, and it seems unlikely that motile 

 reproductive cells would occur in this sub-family. 



Fig. 125. A C, Lagerheimia gene- 

 vensis Cbod. ; A, vegetative cell; 

 7? and C, formation of avito- 

 spores (auts), x about 850 (after 

 Chodat). D and E, L. genevensis 

 var. sitbglobosa (Lemm.) Chod. ; 



D, x 520 (after Lemmermann) ; 



E, x 450. F and G, L. breviseta 

 (W. * G. S. West), x450. H 

 and 7, L. ciliata (Lagerh.) Chod. 

 var. amphitricfmfa&gerh.) Chod., 

 x450. 



The type genus, Micractinium Fresenius ('56 '58) had been for years entirely over- 

 looked until Wille ('09 B) brought it to light again, although the original description and 

 figures were quite good. The present author agrees with Wille's grouping of the recently 

 described forms. 



The genera are: Micractinium Fresenius, 1856 58 [inclus. Archerina Lankester, 1885 ; 

 Phythelios Frenzel, 1891 ; Golenkinia Chodat, 1894 ; Ricliteriella Lemmermann, 1896] ; 

 Acanthosphsera Lemmermann, 1898; Meringospheera Lohmann, 1908; ? Echinosphxridium 

 Lemmermann, 1904 ; Lagerheimia (De Torn') Chodat, 1895 [inclus. Tetraceras Chodat, 



