154 ^ CENTURY OF PROGRESS IN THE NATURAL SCIENCES 



observations have been confirmed by Cliolnolvv (1933b) and more particularly 

 by Iyengar and Subralimanyan (1942, 194-1), Stosch (1951a), and Geitler 

 (1952), It would seem therefore that the Centrales and Pennales are not as 

 remote from each other as has been supposed. 



In 1897 G. Murray observed in certain marine members of the order Cen- 

 trales rounded protoplasmic bodies which he interpreted as reproductive cells. 

 Since then these so-called microspores have been observed by a number of inves- 

 tigators in various marine as well as freshwater Centrales. In some instances 

 the microspores are provided with two lateral and in others with two terminal 

 flagella of equal length. Stosch (1951a) observed with certainty only one flagellum. 

 It has been thought that these microspores are gametes but actual proof of this 

 was not forthcoming until Stosch (1951a, 1951b) and Geitler (1952) showed 

 that in some species they are actually male gametes. For a review of the litera- 

 ture on the microspores reference should be made to the works of Karsten ( 1928, 

 pp. 167-175), Fritsch (1935, pp. 633-637), Subralimanyan (1946), and 

 Stosch (1951a). 



Utilizing a concept introduced into the classification of diatoms by Grunow 

 in 1860, Kirchner (1878) and Schiitt (1896) divided these organisms into two 

 groups, called Circulares and Bilaterales by Kirchner (p. 41) and Centricae 

 and Pennatae by Schiitt, on the basis of the shape and sjonmetry relations of 

 the valves. West (1904) elevated these two groups to the rank of order, ac- 

 cepting Schiitt 's designations, and Karsten (1928) changed the names to Cen- 

 trales and Pennales. Rabenhorst (1853) was the first to consider the diatoms 

 as constituting an independent class of algae, which he (1864) named Diatomo- 

 phyceae. The currently accepted name, Bacillariophyceae, was proposed by 

 Fritsch (1935, p. 7). Engler and Gilg (1924, p. 13) and Karsten (1928) have 

 elevated the group to the rank of phylum (Bacillariophyta) but in general phy- 

 cologists have adhered to the interpretation of Pascher (1914, 1921), who, largely 

 on the basis of the formation of endoplasmatic cysts in certain Centrales (first 

 correctly interpreted by Schiitt in 1888) comparable to those of Chrysophyceae 

 and Xanthophyceae, has related them to the latter two classes. The advantages 

 of the present system of classification of the diatoms, which is based largely 

 on the characters presented by the siliceous shell, is that it is applicable to the 

 many fossil representatives (which are of considerable economic importance) as 

 well as the living forms. The arrangement presented below is essentially that 

 of Hustedt (1930). 



Class Bacillariophyceae Fritsch (1935, p. 7) 



Syn.: Class Diatomophyceae Rabenhorst (1S64, p. 2); Order Pyritophyceae Stizen- 

 berger (1860, p. 23) ; Division Bacillariophyta Engler et Gilg (1924, p. 13) 

 Order CENTRALES (Schiitt) West orth. mut. Karsten (1928, p. 201) 



Family Coscinodiscaceae Kiitzing orth. mut. De Toni (1890, p. 915) 

 Syn.: Thaumatodiscaceae Cleve orth. mut. De Toni (1894, p. 1010); 

 Melosiraceae Kiitzing orth. mut. De Toni (1890, p. 913); Xanthio- 

 pyxidaceae (Petit) De Toni (1890, p. 914) ; Discaceae Schutt orth. mut. 

 Karsten (1928, p, 201) 

 Family Asterolampraceae H. L. Smith orth. mut. De Toni (1890, p. 919) 

 Syn.: Heliopeltaceae H. L. Smith orth. mut. De Toni (1890, p. 918); 

 Actinodiscaceae (Schutt) Hustedt (1930, p. 56) 

 Family Eupodiscaceae Kutzing orth. mut. De Toni (1890, p. 916) 



