436 Ecology 



occur in the deep, uncontaininated lakes, yet requires thorough investigation, 

 and it would appear that its complete scientific explanation can only be 

 obtained by simultaneous biological and chemical observations extending over 

 a considerable period of time (W. & G. S. W., '12) 1 . Anab&iia Lemmermanni 

 sometimes occurs in abundance, its spores forming deep blue-green floating 

 clusters which may give a decided colour to surface water of an entire lake 

 (Lemmermann, '03 ; W, & G. S. W., '06). Most of the plankton-species of 

 Anab&na have spirally coiled filaments. Anabt&na is also an important genus 

 in the great African Lakes (consult Schmidle, '02 ; G. S. W., '07 ; Ostenfeld, 

 '08 ; Virieux, '13), more especially in Tanganyika and in Nyasa. In these 

 lakes a curious coiled type occurs in which the filaments are extremely 

 short and terminated at each end by a heterocyst (consult fig. 19 A E). 

 Woloszynska ('12) has also observed these same forms in the plankton of 

 lakes in Java and suggests that they should be placed in a special section of 

 the genus Anab&iwpsis. 



Several spirally twisted species of Lyngbya also occur in the plankton, 

 though never in such quantity as to be dominant. The most notable are 

 L. Lagerheimii, L. contorta and L. circumcreta (fig. 19 F H}. 



PERIDINIE/E. The one Peridinian which is ubiquitous throughout the 

 freshwater phytoplankton of the world is Ceratium hirundinella. In the 

 colder temperate countries it is a summer form, completely disappearing from 

 the plankton in the winter months, entering into an encysted state (fig. 54 E) 

 on the advent of cold weather. It usually reappears about the middle of 

 spring. In warmer regions it is perennial (Entz, '04 ; Lemmermann, '08). 

 In the larger and deeper lakes this organism is rarely very abundant, but it 

 attains to a position of dominance in some of the shallower lakes. Although 

 found throughout the world it does not occur in every lake even in the same 

 area. It is entirely absent from Wastwater, although frequent in many of 

 the other English Lakes, and so far as is known it is quite absent from the 

 larger lakes of North Wales. During thirteen months' continuous observations 

 on the plankton of the Yan Yean Reservoir, Victoria, no trace of Ceratium 

 hirundinella could be discovered, and yet it was not infrequent in the 

 Toorourong Reservoir from which the main water-supply of the Yan Yean is 

 derived by aqueduct (G. S. W., '09 B). The organism is exceedingly variable 

 in size and in the length and number of its horns, the many forms having 

 been well described and figured (consult Lemmermann, '04 ; '10 ; W. & G. S. W., 

 '05; '06; Bachmann, '07). Two of these forms stand out conspicuously from 

 the others. One is var. brachyceras (v. Daday) Ostenfeld ('09), a stunted form 

 characteristic of Victoria Nyanza, and the other a form with a curiously 



1 It has been suggested by Snow ('03) as a result of both observations in nature and cultural 

 experiments that the appearance of ' water-bloom ' is possibly due to the presence of an unusual 

 amount of dead organic matter in the water. 



