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 cellulose or mucilaginous investment which has frequently the form of two or more minute armour-like plates of a limiting membrane. The chromatophores are green, yellow, brown or colourless. Simple division is the usual method of reproduction, 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 rôle 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 slime[165]. 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 Delitzsch in Saxony. A comparison of Ehrenberg’s figures of the fossil species Peridinium pyrophorum Ehrenb.[166], with those of the recent species Peridinium divergens Ehrenb., as given by Schütt[167] and other writers, brings out clearly the very close resemblance if not identity of the two forms. Bütschli[168] in his account of the Dinoflagellata in Bronn’s Thier-Reich confirms Ehrenberg’s determination of Peridinium pyrophorum, and points out its striking agreement with the recent species.