CH. VII] PERIDINIALES. 117 



coenocytic plants may show well-marked external differen- 

 tiation of the thallus into members or parts subserving different 

 functions. 



A similar wide range is covered by the methods of repro- 

 duction among thallophytes. 



I. PERIDINIALES. 



The organisms included under this head are of little 

 importance from a palaeontological point of view, but a brief 

 reference may be made to them as a section of the Thallophyta. 



The Peridiniales include very small single-celled organisms 

 which have often been described as occupying a position on the 

 borderland between animals and plants, lying on the " shadowy 

 boundary between animal and vegetable life." The individuals 

 are rarely naked, more frequently they are covered with a cellu- 

 lose or mucilaginous investment which has frequently the form 

 of two or more minute armour-like plates of a limiting mem- 

 brane. The chromatophores are green, yellow, brown or 

 colourless. Simple division is the usual method of reproduc- 

 tion, but spores have been described as occurring in some 

 species. The motile forms are provided with cilia. The 

 Peridiniaceae, a section of the Peridiniales, are regarded as 

 nearly related to the Diatoms. 



The Peridiniales play an important role in the Plankton 

 flora of the sea and freshwater lakes, and have a world-wide 

 distribution. In the narrative of the Challenger cruise they 

 are described as occasionally filling the tow-nets with a yellow 

 coloured slimed Some genera, such as Ceratium, are found in 

 enormous numbers off the British coast. 



As an example of the occurrence of fossil representatives of 

 the Peridiniaceae reference may be made to one of two species 

 of Peridinium described by Ehrenberg in 1836. These were 

 found in a siliceous rock described as Cretaceous in age from 

 Delitszch in Saxony. A comparison of Ehrenbergs figures of 

 the fossil species Peridiniam pyrophorum Ehrenb.^ with those 



1 Challenger (85) p. 934. 



2 Ehrenberg (36) p. 117, PI. i. figs. 1 and 4, and Ehrenberg (64) PI. xxxvii. 

 fig. vii. 



