224 NATURAL HISTORY OF BHOMPSTON POND. 



Peridinece seems plentiful in most parts of the pond. It is 

 a curious little organism bearing two flagella, one of which is 

 a pulsellum and the other is curved round the cell in a groove, 

 which is clearly visible with a high power. The cell contains 

 a nucleus and brownish chromatophores. 



The Desmidiaceae are living in the pond in great profusion. 

 They are usually unicellular organisms, with a constriction 

 lodging the nucleus in the centre, on either side of which is a 

 bright green chromatophore with pyrenoids. Closterium 

 moniliferum (fig. 82), however, has no constriction, and the 

 chromatophore is composed of fused plates with pyrenoids (a). 

 One of the characteristic features of these beautiful yet 

 lowly plants is the tiny vacuoles (fig. 83a), one in each end of 

 the cell. When a healthy specimen of Closterium is examined 

 under a high power, these vacuoles are seen to contain about 

 twenty extremely minute bodies (b), probably crystals of 

 calcium sulphate, which are constantly oscillating (Brownian 

 movement). Closterium lunula, a handsome moon-shaped 

 species of about l-50th of an inch in length, is found in the mud 

 in some parts of the pond, but is less common than the 

 straighter and more slender form Closterium acerosum (fig. 84). 

 In neither of the latter species is there any constriction 

 between the two segments. Another common form is 

 Cosmarium tetraophthalmum (fig. 85), with the semi-circular 

 segments deeply divided and the edges crenate. It is about 

 1 -200th in. in length. Other species are present in the pond. 

 These interesting plants are able to respond to the stimulus of 

 light, and by means of protoplasmic threads they push 

 themselves along. 



The filamentous forms of Conjugate are to be found in 

 masses in several places. Mougeotia (fig. 86) is of special 

 interest, since the chromatophore (a) is plate-like and the cell 

 adjusts its position to its immediate needs, ; thus in very 

 strong light the chloroplast is turned with its thin edge 

 towards the light (b), while in weak light the broad 

 surface is presented (c). It is quite easy to watch them turn 

 over under a IOAV power the process only taking a few 



