and their relation to existing Animals. 117 



cific forms than is contained in some classes of animals. " It 

 would be almost impossible," continues Ehrenberg, " to bring 

 280 different and distinct forms under a general view, were I not 

 to avail myself of a physiological systematical arrangement." 



The minute and elegant shells of the Polycystina undergo no 

 change upon being immersed and boiled in hydrochloric acid ; 

 on the contrary, they are separated from all adherent foreign 

 matter and become as transparent as crystal. 



In 1838 Ehrenberg classed provisionally the few fossil forms 

 of Polycystina then known, under a separate division of Poly- 

 gastrica with siliceous shells or shields, in expectation that the 

 examination of living forms would point out their true place. 

 But the very distinct and beautiful forms recently discovered, 

 amounting to several hundreds, admit no longer of such an ar- 

 rangement. The formation of these animalcules is very peculiar, 

 and differs from Bacillaria not only in their external form, but 

 also in their internal structure. The reasons which have led 

 Ehrenberg to such a consideration are the following : — 



" If it were possible, in a philosophical point of view, to imagine 

 the existence of Mollusca with siliceous shells among such as possess 

 shells of a calcareous structure, or to ascribe to nearly related ani- 

 mals, to some a transudation of siliceous matter, to others a calca- 

 reous nature, such a supposition would be liable to great objections. 

 The laws intimately connected with the principles of life prescribe the 

 transudation of phosphate of lime from the bones of man and verte- 

 brated animals, and carbonate of lime from the shells of Mollusca 

 and skeletons of Polypi. A human being, or a vertebrate animal 

 with siliceous bones, appears possible if considered in a logical point 

 of view ; but reflections based upon the laws of natural history de- 

 nounce it as inconsistent, if not illogical. Similar in effect would be 

 the idea of a mollusk with a siliceous shell, or a Polythalamia of such a 

 structure — similar to a sickly or anomalous individual of that divi- 

 sion ; it might indeed be compared to a stone of leather or a medal 

 of wood. 



" If we except the Polygastrica, such a physiological law separates 

 the Polycystina with siliceous shells from all classes of crustaceous 

 and vertebrated animals. It would be impossible to assimilate these 

 constant and well-defined forms to individuals of a sickly organiza- 

 tion, as we do not possess forms of a similar structure that might be 

 considered as existing in a perfect and healthy state, to serve as a 

 standard for comparison. 



" It remains now to investigate, whether these minute normal 

 bodies of organic nature are of an independent organization, or whe- 

 ther they are merely parts of organic forms ; whether their structure 

 is of vegetable or animal origin, or whether they are forms of crystals. 



"As we are already acquainted with several instances of living forms 

 of that structure which possess intestines, although their examination 

 has not been quite perfect, the idea of crystallization is rejected as 



