THE MICROSCOPE IN HISTOLOGY AND BOTANY. 141 



of great interest. Generally the cells are independent, 

 but a filament is sometimes formed by binary subdivision. 

 Their symmetrical shape, and frequently spinous projec- 

 tions and peculiar movements, render them beautiful ob- 

 jects. By conjugation a spore-cell or sporangium is pro- 

 duced, which in some species is spinous, and resembles 

 certain fossil remains in flint, which have been described 

 as animalcules under the name of Xanthidia. 



The family of Diatomacece affords more occupation to 

 microscopists than other protophytes. Like the Desmids, 

 they are simple cells with a firm external coating, but in 

 Diatoms this coating is so penetrated with silex, that a 

 cast of the frustule is left after the removal of the organic 

 matter. Reference has already been made to the number 

 of these organisms in a fossil state, as well as to their 

 utility as tests of the defining power of microscopic object- 

 glasses. 



Some species inhabit the sea, and others fresh water. 

 They are so numerous that scarcely a ditch or cistern is 

 free from specimens, and they multiply so rapidly as to 

 actually diminish the depth of channels and block up 

 harbors. They may be sought for in the slimy masses 

 attached to rocks and plants in water, in the scum of the 

 surface, in mud or sand, in guano, in the stomachs of 

 molluscs, etc., and on sea- weeds. 



To separate the shields or siliceous frustules from foreign 

 matter, either fresh or fossil, they should be washed sev- 

 eral times in water, and the sediment allowed to subside. 

 The deposit should then be treated in a test-tube with 

 hydrochloric acid, sometimes aided by heat. This should 

 be repeated as often as any effect is produced, and then 

 the sediment should be boiled in strong nitric acid, and 

 washed several times in water. They may be mounted 

 dry or in balsam. 



The classification of Diatoms is not yet perfected, but 

 Muller's type slides, containing from one hundred to five 



