182 SECTIONAL ADDRESSES. 



one cilium has so far been recognised in quite a number of the Hetero- 

 kontae, and here too a suppression of the second is possible. 



Variation in number of cilia is also a feature in the Isokontae, where 

 the dikontan and tetrakontan types are traceable throughout the class, 

 and quite recently a uniciliate member (Chloroceras) has been described 

 by Schiller 1 " ; this form is particularly interesting because occasional 

 rare individuals show two cilia. Among the Polyblepharidaceae organisms 

 are known with up to eight cilia which presumably result from multipli- 

 cation. • These facts demonstrate the risk of basing a separate class, 

 Stephanokontae, on the occurrence in the Oedogoniales of swarmers with 

 numerous cilia. Moreover, they show that, although cilial characters 

 are of undoubted value in the distinction of the main classes of Algae, 

 the point must not be stretched too far and must be supported by other 

 features. 



The parallel development, evident in Isokontae, Heterokontae, and 

 Chrysophyceae, is recognisable, though not quite so markedly, also in 

 other classes of Protophyta. One further striking instance may be men- 

 tioned. The Peridinieae (Dinoflagellata) are a very distinct and rather 

 specialised class of motile forms, abundant in freshwater and marine 

 plankton, though on the whole more strongly represented in the sea. 

 Their most striking characteristic lies in the division of the body of the 

 cell into two usually slightly unequal, apical and antapical, halves by 

 a transverse furrow harbouring one cilium, while the other trails out 

 behind into the water. There are usually numerous discoid chromato- 

 phores which are commonly dark yellow or brown ; .a number of special 

 pigments (peridinin, chlorophyllin, &c.) have been extracted from them. 

 The reserves are stored as starch and oil. The nucleus is usually large 

 and conspicuous, and shows either a granular structure or contains 

 numerous fine threads. 



It was in 1912 that Klebs 19 described a number of forms that were 

 clearly algal and chlorococcoid members of this class. His Hypnodinium 

 shows the derivation clearly ; it consists of large motionless spherical 

 cells provided with a firm membrane and possessed of the chromato- 

 phores and nuclei characteristic of the class. When reproduction takes 

 place, the protoplast contracts somewhat and develops the distinctive 

 furrows, but this is followed by division without resort to a motile phase. 

 The two daughter-cells show no traces of furrows until a fresh division 

 is initiated. In Phytodinium no such furrow-formation is observed and 

 with it the last indication of the motile phase has disappeared. It is of 

 interest that, among these chlorococcoid Peridinieae, Klebs distinguished 

 one tetrahedral form (Tetradinium) recalling in outward shape the 

 genus Tetraedron among the Isokontae and Pseudotetraedron among the 

 Heterokontae. In 1914 Pascher 20 briefly described a slightly branched 

 filamentous Alga, Dinothrix, which is stated to have the discoid yellow- 

 brown chromatophores and the large nucleus of the Peridinieae and to 

 reproduce by swarmers resembling a Gymnodinium, one of the simplest 

 of the motile types. Unfortunately no further description or figure of 



18 Osterr. Bot. Zeitschr. lxxvi, 1927, p. 1. 



19 Verhandl. Nat.-Med. Ver. Heidelberg, xi, 1912, p. 369. 



20 Ber. Deutsch. Bot. Ges. xxxii, 1914, p. 160. 



