304 DESCEiPTiON OF \^Poly gastric a. 



Ou the other hand, the following facts are in favonr of their 

 vegetable natiu-e : — 



1 . The great resemblance of compound forms to Algae, and their 

 develoj^ment by fission. There are, indeed, compound Infusoiiaj as 

 Monad-masses and Polypes, but the former are very questionable 

 animals, and the latter have this essential distinction, that the indi- 

 vidual animal lives without (external to) its habitation, and moves 

 freely ; whereas such Naviculce as Encyonema, ScJiizonema, and Mi- 

 cr omega, and similar genera, grow within the enclosing substance, 

 building themselves up like the cells in the stem of a plant, so 

 vegetating here only as cells. In like manner, the individuals of 

 Fragilaria, Mehsira, Himantidium, &c., are steadily fixed, and 

 tuiable to exhibit animal motion. 



2. The inner soft, organic parts, which I have designated gonimic 

 substance, possess, as well in their chemical nature as in their 

 development, peculiarities akin to those met with in the cell-contents 

 of confer void Algae. 



This relation is most clearly seen in the genus Mehsira and its 

 allied forms, which, not only in form, but also in the chemical com- 

 ponents of their contained matter (since the presence of chlorophyll 

 is common to all Diatomece), are closely allied to the confervoid Algae. 



3. The development of seeds, or young, (as Kiitzing represents it) 

 occurs here as in undoubted Algae, but never as in true animals. 



4. The Biato^necG, and especially the free, moving Nwviculce, 

 developej in the sun's rays, an appreciable quantity of oxygen, like all 

 admitted plants. 



The evolution of oxygen, indeed, occurs in gi-een Monads and 

 Euglena, but this affords no argument for the animality of the 

 Diatomece, but renders the animal nature of those Infusoria themselves 

 veiy doubtful ; and the more so, as recent observations conifirm the 

 idea of the origin of the lower plants themselves, from Monads and 

 Ewjhna, (page 18.) Wherefore, aU these comparisons serve to 

 favour the belief in the vegetable nature of Diatomece, (Diekiesels- 

 chaligen, Bacillarien odcr Diatomeae, Von Dr. F. T. Kiitzing, 

 Nordhausen, 1844.) Some English writers have acceded to the 

 opinion that tlie Diatomm arc vegetables, from the above views 



