The Algæ form three principal groups: 1. the _jointless_, as the Fuci, the Dulses, Tangles, and Lavers: 2. the _jointed_, which are composed of thread-like articulated tubes; such are the fresh-water Confervæ: 3. the _disjointed_, or Brittle-worts, so called from their spontaneous self-division, which is in some kinds complete, in others only partial; and these, by separating transversely, and leaving each cell or frustule attached at the angles, produce those beautiful chains of angular green transparent cases, so constantly seen under the microscope when substances from fresh-water streams or lakes are submitted to examination. As many of these forms are endowed with spontaneous motion, and possess other properties common to animal organization, it is not surprising that their vegetable nature was doubted, and that even so profound a naturalist as M. Ehrenberg placed them in the animal kingdom: the greater number being comprised in his family of _Bacillariæ_, were described in the former edition of this work, as Infusoria or Animalcules; in conformity with the classification of the illustrious microscopist, whose splendid works and indefatigable labours have so greatly promoted the advancement of microscopical investigation.[61] [61] The whole of the objects called _Infusoria_ in the first edition of "The Medals of Creation" belong to various kinds of Diatomaceæ. [Sidenote: FOSSIL DIATOMACEÆ.] These minute vegetable organisms are placed by botanists in two tribes, the _Diatomaceæ_ or the _Brittle-worts_, and the _Desmidieæ_. The latter are exclusively inhabitants of fresh-water, while a large proportion of the former are marine plants. Some naturalists (M. Brébisson) restrict the name _Diatomaceæ_ to those species which secrete siliceous envelopes; and that of _Desmidieæ_ to those whose structures are not siliceous, and are reducible by heat to carbon. As the durable parts of these plants alone concern the geologist, the name _Diatomaceæ_ will be employed as a general term in reference to their fossil remains.