DIATOMACE^ 189 



under Clilorophycece, notably the Dcsmidicce, the 

 Peridiniece (which have also a peculiar colouring- 

 matter), and, it may eventually prove, the calcareous 

 Coccospheres and Rhabdospheres. The Diatoms 

 have a very extensive literature of their own, from 

 the fact that their great beauty and universal dis- 

 tribution have made them in a singular degree the 

 objects of special study. 



General Characters. The plants are very minute 

 unicellular, with the walls strongly silicified, and 

 composed of two overlapping halves or shells called 

 valves. The valves overlap, the one above the other at 

 the edge, much as the lid of a cardboard box overlaps 

 the box itself, and this overlapping edge is called the 

 girdle. Each individual, consisting of two valves, is 

 called frfrustule in the special literature of the sub- 

 ject. The colouring-matter is chlorophyll masked 

 by a brownish substance called diatomine, readily 

 soluble in alcohol, and resembling the peculiar 

 colouring-matter of the Phccophycccc. Many Diatoms 

 are endowed with a power of independent movement. 

 The reproduction affords highly distinctive charac- 

 ters. After a series of successive bipartitions, in- 

 volving a gradual diminution in the size of each new 

 generation, the original dimensions are regained by 

 the formation of an auxospore by various methods, 

 some of them involving conjugation, but not 

 between motile gametes. 



The Thallus. The individual Diatoms live either 

 singly or in chains, and they may be wholly free, or 

 borne on gelatinous stalks, or enveloped in a 

 gelatinous mass. The fresh -water forms do not 



