100 SCIENCE PROGRESS 



domonads themselves have been shown to present many simi- 

 larities to certain of the living Flagellates (Cryptomonadineae), so 

 that the origin of the Isokontae from Flagellata is fairly well estab- 

 lished. As groups equivalent to the Isokontae, certain author- 

 ities (Bohlin 8 and 9, Blackman and Tansley 6) have established 

 the Heterokontae, the Stephanokontae (CEdogoniaceae), and the 

 Akontae (Conjugatae). There is little doubt about the origin of 

 the first of these groups from a Flagellate ancestry (cf. however 

 Wille 70a, p. 2) distinct from that of the Isokontae, but we cannot 

 say the same for the other two. The Stephanokontae are a small 

 group, including but a single order, the Qildogoniaceae, the best- 

 known representatives of which are the genera GLdogonmni and 

 Bulbochcvte (Pringsheim 56, Hirn 34). Among the numerous 

 peculiarities of the CEdogoniaceae is the production of large 

 zoospores (cf. also Pascher 55), formed singly from the cells of 

 the filament and provided with a crown of cilia inserted a little 

 way from the front end and around the base of a prominent 

 colourless beak (fig. 4, c). The establishment of the group of 

 the Stephanokontae is based on these peculiarities of the zoo- 

 spore (the androspores and spermatozoids show similar features), 

 but it must be borne in mind that the large size of the (Edo- 

 goniaceous zoospore as compared with that of the Isokontae may 

 well have brought with it an increase in the number of cilia, 

 and that in the zoospores of the Isokontae the cilia also fre- 

 quently arise from the base of a small colourless papilla at the 

 anterior end. The zoospores alone, therefore, would not give 

 too much weight to the view of the independent origin of the 

 QLdogoniace^, particularly as in Pc>/)'Z'/^/)/7<7r/5 (fig. r, l, p. 629) we 

 have among undoubted Isokontan forms a motile organism with a 

 considerable number of cilia. Moreover, we have at present no 

 knowledge of a Flagellate showing any marked resemblance to 

 the (Edogoniaceous zoospore. The origin of the Stephanokontae 

 from a distinct Flagellate group is therefore highly doubtful. 

 Nevertheless, the group presents certain marked peculiarities 

 that distinguish it rather sharply from the Isokontae and which 

 might favour a distinct origin more than the characters of the 

 zoospore do. Thus the method of growth and cell-division, 

 which takes place by means of a curious thickening-ring at the 

 upper end of the cells (fig. 4, a) and leads to the formation of 

 the characteristic caps (fig. 4, b), is without direct parallel 

 (Hirn 34); and the same may be said of the dwarf-males (found 



