22 ZOOLOGY. 



is considered ; some of the silicious shields being so small, that, according 

 to Ehrenberg, one hundred millions weigh but a grain, Thej are found in 

 flint, semiopal, bog iron ore, ochre, tripoli (and other polishing minerals 

 v.-hich owe their action to the shields of silicious species), mountain meal, 

 a clay which is eaten in Lapland, and another variety in South America, 

 but which probably merely fills the stomach without affording nourishment. 

 Sometimes these remains form a stone sufficiently light to float in water, and 

 strong enough to be employed in building. In Liiueburg and about Berlin, 

 the infusorial strata are as much as twenty feet deep and twenty miles in 

 extent. Infusorial strata have been discovered by William B. Eogers u])6n 

 the Happahannoc river at Stratford cliffs, on the Potomac, and on James 

 Eiver below City Point ; and in other parts of Virginia. These deposits 

 belong to the miocene formation. 



The extent of the artificial group Infusoria will probably be much reduced 

 as researches are continued. Kiitzing has separated the Jjacillaria and 

 JDiatoineoi as Algce^ and he considers Gaillonclla ferruginca to be a 

 conferva. He thinks some of these organisms have both a vegetable and 

 an animal nature, and that in such simple forms the distinction between 

 animal and vegetable does not exist. 



The observation.s of linger, Plotow, Thuret, and C. Th. Siebold, have 

 thrown doubts upon the animal nature of other Infusoria of the genera 

 Enchelys, Chlamidomonas, Chilomonas, Chnetoglena, and others. These 

 researches seem to show that the spores of Algse are locomotive by means 

 of vibrillip, although linger and Bory would contend that these supposed 

 spores are animal in their nature, but convertible into plants. 



x\gassiz considers many of the Infusoria to be the larvae of worms, etc., 

 and he seems disposed to suppress the entire class by distributing its 

 members among the other classes of animal and vegetable nature. There 

 is reason to believe that supposed species of Leucophra and Difflugia are 

 immature Alcyonellie. jS"everthless, it would be premature to give up the 

 entire class until a greater number of the more distinct forms are 

 ascertained to be larvsjs, because, whilst we admit that individual species 

 may have been described under several distinct names, the number of 

 infusorial species seems too great for the comparatively small number of 

 worms, &c., likely to be derived from them. 



The name Infusoria is adopted here on account of the heterogeneous 

 contents of the division ; but if future observation renders it probable that 

 the removal of portions will still leave a distinct group incapable of union 

 with other classes, these may take the more appropriate name of Protozoa^ 

 sometimes applied to the entire group ; and its symbol, to extend the views 

 of Agassiz, will be a circle. 



Professor J, W. I?ayley of the Military Academy, West Point, is the chief 

 American authority uj^on the Infusoria. Most of his papers may be con- 

 sulted in the American Journal of Science. 



We now proceed to the consideration of the species figured in the atlas. 



226 



