146 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



SILICOFLAGELLATOPHYCIDAE 



This subclass includes only six clearly defined genera of marine flagellates. 

 The first representatives of the group to be described were fossil forms that 

 were found by Ehrenberg in 1839 in cretaceous marls from Oran and Sicily. 

 He erected the genus Dictyocha for these fossils and two years later (Ehren- 

 berg, 1841) observed the first living specimens of this genus in water from the 

 North Sea. In subsequent years he described a large number of additional spe- 

 cies as well as a second genus {Mesocena) . 



Ehrenberg believed these organisms to be diatoms. Haeckel (1862) placed 

 them with doubt with the Radiolaria. The group retained its doubtful alliance 

 with the Radiolaria until 1891 when Borgert showed, as a result of a detailed 

 study of living specimens of DistepJianus speculum, that they differed strikingly 

 from Radiolaria. He observed the occurrence of brown plastids in the cells and 

 also established for the first time that the cells owed their motility to the pres- 

 ence of one (Distephanus) or two {Ehria) flagella. Borgert consequently con- 

 sidered these organisms as an autonomous group of flagellates for which he 

 (1891, p. 661) proposed the name Silicoflagellata. 



On the basis of Borgert's findings, Haeckel in 189-4 (p. 126) classified these 

 organisms with the algae. Engler (1903) considered them (with a query) as 

 constituting an independent phylum of thallophytes. 



Lemmermann (1901a, 1901b) gave the first systematic treatment of the group 

 and the present system is still essentially that proposed by him. Largely on 

 the basis of skeletal structure he divided the group into two orders: (1) the 

 Siphonotestales, which are uniflagellate and in which the skeleton is composed 

 of hollow siliceous beams, and (2) the Stereotestales, which are biflagellate and 

 in which the siliceous framework of the skeleton is solid. Each of these orders 

 received a single family. Although the,y appeared to constitute a clearly de- 

 marked group, Lemmermann (1901b, p. 254) thought the silicoflagellates might 

 be related to certain of the other groups of flagellates. 



Pascher (1912a, p. 193) brought attention to the correspondence between 

 the skeletons of silicoflagellates and the cysts of Chrysophyceae and hence allied 

 these groups. With the notable exception of Schulz (1928) and Gemeinhardt 

 (1930, 1931), who believe that the silicoflagellates constitute an autonomous 

 class, the majority of students of the group concur with Pascher in relating 

 them to the Chrysophyceae. Hovasse (1932) is of the opinion that the Ebria- 

 ceae (which are heterotrophic) may be more nearly related to the Radiolaria 

 or certain Dinophyceae than to the Silicoflagellatophycidae. 



The most comprehensive treatment of the group is that given by Gemein- 

 hardt (1930) in Rabenhorst's Kryptogamen-Flova . . . Almost half the known spe- 

 cies of the world are known only from fossils. In 1931 Gemeinhardt published 

 a valuable account of the silicoflagellates collected during the German South 

 Polar Expedition of 1901-1903. 



The systematic arrangement of the Chrysophyceae presented below departs 

 in certain respects from that of Pascher (1931). The present arrangement is 

 a synthesis of the systems of Pascher (1931), Fritsch (1935), Huber-Pestalozzi 

 (1941), and Smith (1950). It should be emphasized, however, that our knowl- 



