120 THE PLANTS 



Peridiniales. This group includes a very large pro- 

 portion of the phytoplankton. The Diatoms seem to 

 be the chief constituent of the phytoplankton in colder 

 waters, while the Peridiniales seem the more con- 

 spicuous group in warmer seas. 



The members of this group are easily distinguished 

 from the Diatoms. Their wall is generally less trans- 

 parent, so that the contents are not so clearly seen ; 

 and it is marked out, not into two portions merely, 

 but into a number of angular plates fitting closely 

 together. The plates may be smooth, but are usually 

 marked with a number of pits. The walls consist not 

 of silica, but of cellulose, the substance which is the 

 main constituent of paper. Running round the body 

 of the plant is found a well-marked groove, the hori- 

 zontal furrow, which is connected at one point with a 

 pore in the shell. From this aperture there grows out 

 from the living substance of the body a long, whiplike 

 swimming organ, the flagellum, which projects freely 

 into the water. Arising from the same aperture is 

 another flagellum, which lies in the horizontal groove ; 

 these flagella can only be seen with considerable diffi- 

 culty. The position of the horizontal groove varies 

 much in different forms. In some cases, as in Ceratium 

 (Fig. 38), it divides the body into two equal halves. 

 In other cases the furrow is found quite at one end of 

 the body ; it may also occupy all intermediate 

 positions. Running up from the pore there is usually 

 a short longitudinal furrow, which is, however, far 

 less conspicuous than the horizontal one. 



The form of the body is wonderfully diverse in this 

 group, Nature seeming to have given herself a free 

 hand in the development of the most bizarre shapes. 

 Some possess long, thin spines ; in others the body is 

 enormously elongated, or rounded and s\\ ollen ; others 



