THE PROTOZOA 



complicated by the presence of spines, wing-like processes, and other 

 appendages, or they may be pitted by minute depressions or pores. 

 After the death of the animal the plates can, as a rule, be separated 

 by gentle pressure. The substance of the shell is not true plant cel- 

 lulose, but a modification, the exact nature of which has not been 

 definitely determined. 



The furrows in the shells of the Dinoflagellidia, in which the two 

 flagella lie, are perhaps the most characteristic feature of these forms. 



B 



Fig. 62. Phacotus lenticularis Ehr. [BUTSCHLI.] 

 A. Individual within its bivalved shell. B. Spore-forming individual. 



One runs across the organism, while the other, which may often, how- 

 ever, be obliterated, is at right angles to this, usually in the direction 

 of the longitudinal axis. There may be one, two, or many transverse 

 furrows, the number determining the family to which the organism 

 belongs. In the genus Hemidinium, the single transverse furrow 

 begins on the ventral side and runs as far as the middle of the dorsal 

 side, where it disappears. In the genera Gymnodinium, Glenodininm, 

 and Peridinium, it runs completely around the organism ; while in 

 Ceratium it may be broken in its course (Fig. 64).' The longitudinal 

 furrow, on the other hand, is invariably confined to the ventral side, 

 usually to the lower half, but in some cases (Glenodinium, Peridinium] 

 it traverses the cross furrow and stretches some distance along the 



