CLASSIFICATION 



85 



tant of these are the Peridineae and the Diatoms. These organisms, 

 although of minute, often microscopic, size, are of enormous impor- 

 tance, as they are the principal green organisms of the plankton, 

 and are the original source of food for nearly all marine animal life. 



Structure of Peridineae 



The Peridineae (Fig. 59) show much resemblance to some of the Flagellata, 

 with which they are probably related. Like them, they are provided with two 

 flagella, which are, however, usually inserted laterally. They generally show 

 a membrane of cellulose which is 

 in the more specialized forms com- 

 posed of a number of sculptured 

 plates joined together. The lowest 

 order, Gyrnnodiniacese, are either 

 naked cells, like most Flagellata, 

 or they possess a simple cellulose 

 or gelatinous membrane. In the 

 higher forms the membrane shows 

 a division into two valves, or pieces 

 which fit together, and in this 

 respect the Peridineae much resem- 

 ble the Diatomaceae. 



Some of the Peridineae are desti- 

 tute of chroinatophores, but these 

 are usually present. They may be 

 pure green, or there may be present 

 a yellow or brown pigment, as in 

 the Diatoms, this being especially 

 the case with the marine forms. 



Reproduction. Reproduction is FlG - 59. A, Hemidinium nasutum (X260). 

 principally by simple fission, but > Peridinium divert/ens (x350). C, 

 sometimes there is the formation of Pyocystis lunula (x 350) (A, after 



STEIN; B, C, after SCHUTT.) 

 zoospores, or motile reproductive 



cells by internal cell-division. No sexual reproduction has been observed, but 

 it is probable that there is a conjugation of the free-swimining cells. 



Distribution. The Peridineae are most abundant floating on the surface of 

 the sea, but they may also occur in fresh water. Some of the marine species, 

 e.g. Ceratium tripos, are joined in chains, but more commonly they are isolated 

 cells. Some of the Peridineae, like Pyrocystis noctiluca, are among the impor- 

 tant phosphorescent organisms of the ocean. 



Affinities of Peridineae. The Peridineae are probably related to 

 the Flagellata, but they also show resemblances to the Volvocaceae, 

 the lowest of the Green Algae, and also to the characteristic group 

 of Seaweeds, the Brown Algae. The zoospores of the latter are strik- 

 ingly similar to some of the simpler brown Peridineae, and suggest 

 a real relationship between the two groups. In their color, and the 

 structure of the membrane, there is a suggestion of the Diatomaceae. 

 It seems likely, then, that the Peridineae are a very primitive group 

 of organisms, with affinities in several directions. 



