38 CLASS I. 
life in the direction of the smallest space. There are infusories 
(monads) having a mean diameter of yg... so Of a line, 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}. 
Of the minute animals that are usually comprehended under the 
term infusories, HHRENBERG’S investigations led him to distinguish 
two classes, which he named Polygastrica and Rotatoria?. ‘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 im 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 3 of an English inch. 
* For the literature vid. Srmpotp and Srannius, Lehrb. der vergleich. Anatomie, 1. 
Abtheilung. Berlin. 1845. 8vo. s. 7. 
2 
