282 



THE MICROSCOPE. 



have much the appearance of condensed gelatine ; but as 

 they increase in size, the internal fluid acquires a green 

 tint, which is at first very faint, but soon becomes darker ; 



Fig. 154. 



7, Sphcerozosma vertebratum. 8, 9, XantMdiue. 10, X. armatum. 11, Cosmo- 

 im crenatum. 13, 17, Sporangia of Cosmarium. 14, X. fasiculatum. 1, 

 Arthrodcsmus convergens. 15, Sto-urastrumtumidum. IGfStaurastrumdilitatum. 



at length it assumes a granular state. At the same time 

 the new segments increase in size, and obtain their normal 

 figure ] the covering in some species shows the presence 

 of puncta or granules. In Xanthidiwm and Staurastrum 

 the spines and processes make their appearance last, 

 beginning as mere tubercles, and then lengthening until 

 they attain their perfect form and size, armed with seta3 ; 

 but complete separation frequently occurs before the whole 

 process is completed. This singular process is repeated 

 again and again, so that the older segments are united 

 successively, as it were, with many generations. When 

 the cells approach maturity, molecular movements may 

 be at times noticed in their contents, precisely similar to 

 what has been described by Agardh and others as occurring 

 in Confervce. This movement has been aptly termed a 

 swarming. All the Desmidiacece are semi-gelatinous. In 

 some the mucus is condensed into a distinct and well-defined 



