38 CLASS I. 



life in the direction of the smallest space. There are infusories 

 (monads) having a mean diameter of ~ a&s f a ^ me J which yet 

 live so closely together, that the intervening space scarcely exceeds 

 their diameter ; a single drop of water measuring a cubic line, if 

 only one-fourth of its space were filled with such animals, would 

 still contain 500 millions of them. 



This minuteness has misled some authors to designate infusories 

 as microscopic animals. We cannot allow to this appellation a 

 preference to that of infusories : magnitude ought not to supply the 

 character of a class of the animal kingdom, or a ground of division. 

 By such an appellation, the union of diminutive species of higher 

 classes of animals with infusories, often practised by older authors, 

 would be justified. 



In determining what is to be understood by infusories, we must 

 look to the whole of their organisation : it requires not many words 

 to shew that the investigation of the organisation of creatures so 

 minute has difficulties to contend with, which even the best optical 

 expedients of our time have only partially removed. For, although 

 every species of infusories be not so small as to escape the naked 

 eye, yet even these are not bigger than two or three millimeters 1 . 

 Of the minute animals that are usually comprehended under the 

 term infusories, EHRENBERG'S investigations led him to distinguish 

 two classes, which he named Polygastrica and Rotatoria 2 . The 

 incontestably greater complexity of structure in the last, the sym- 

 metry of their form and their resemblance to the type of the Articu- 

 lata, suggested to us, as early as 1834, the propriety of separating 

 them entirely from the others a proceeding now approved of by 

 almost all zoologists. 



Consequently, we comprise in the class which now occupies us 

 only those animals which EHRENBERG calls Polygastrica?. We 

 have not, however, adopted that name, for it rests on the opinion 

 that the cavities observable in the interior of these animals are 

 stomachs, which is doubted by many writers : but even if that were 

 admitted, numbers remain in which no such stomachal cavities are 

 to be seen. The class, thus limited, contains animals of very simple 



1 A millimeter is about half a line, or ^ of an English inch. 



a For the literature vid. SIEBOLD and STANNIUS, Lelirb. der vergleich. Anatomic, I. 

 Abtheilung. Berlin. 1845. 8vo. s. 7. 



