34 BRITISH FRESHWATER RHIZOPODA. 
CLASSIFICATION. 
Dusarpin* made one of the earliest attempts at a 
classification of the Protozoa, and in his second order 
of the Infusoria, in which he placed his “ Animauzx 
pourvus dexpansions variable,’ he approached very 
nearly to the classification of the present day. The 
three families into which he divided this order— 
Amibiens, Rhizopodes, and Actinophryens—corres- 
pond closely with (1) the Ameebina, (2) the Conchu- 
lina and Foraminifera, and (3) the Heliozoa. More- 
over he grouped his three families into two sections, 
placing the Amibiens and Rhizopodes in one, and the 
Actinophryens in the other, as we now differentiate 
the Rhizopoda from the Heliozoa. Later, von Siebold + 
divided the Protozoa into two classes, the Rhizopoda 
and the Infusoria, the former including the above 
three families of Dujardin. This obviously natural 
primary division served for a time; but advancing 
knowledge rendered further systematization indis- 
pensable. 
Four classes or main groups of Protozoa are now 
generally recognized, namely, the Sarcodina (with the 
Heliozoa), at the bottom of the scale; the Mastigo- 
phora or flagellates ; the Sporozoa; and the ciliated 
Infusoria. The inter-relations of the Sarcodina, the 
only class with which we are now concerned, remain 
more or less uncertain. Obscurity surrounds the life- 
history of many forms, rendering a perfect division 
into genera and species for the present impracticable. 
Haeckel separated the (supposed) non-nucleated forms 
from the nucleated, or those in which the existence of 
a nucleus had been demonstrated, in order to establish 
* «Hist. Nat. des Zoophytes. Infusoires,’ 1841, p. 126, 
+ ‘Anatomie der Wirbellosen Thiere,’ 1848, p. 3. 
