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These organisms are true animals. Recent observations,^[1] it may be said, throw some doubt over the nature of a few of the simple Rhizopods, but they are far from proving their supposed affinities with plants. And, until further researches compel us to alter our belief, we see no sufficient reason for dissenting from the generally-received opinion, that Amæba and Actinophrys perform all the necessary functions of undoubted animals. With regard to Infusoria, if the possession of a mouth be demanded as an essential condition in all future definitions of this group, we at once get rid of the hosts of Rhizopods and vegetable forms, which some, following Ehrenberg, still associate with the Infusoria properly so called. It yet remains to distinguish these latter from the numerous embryonic forms of higher animals, more particularly of Annulloida, with which, in all probability, they are in many cases confounded.

A good classification of the Protozoa is still a desideratum. Much has yet to be done towards the attainment of a more perfect knowledge both of their structure and development, before even the data necessary for such an object can rightly be perceived. How far, in the present state of inquiry, we seem justified in indicating the outlines of a natural arrangement, may be inferred from the following considerations.

Some of the Protozoa possess, others are destitute of, a mouth. The former are termed Stomatoda, the latter, Astomata.^[2] The Stomatoda include all the Infusoria proper, and the solitary genus Noctiluca. Under Astomata are placed all remaining Protozoa. Whether the six groups of Protozoa Astomata, recognized in the above list, be precisely equivalent to one another, must remain an open question. By J. Müller^[3] the term Rhizopoda has been extended, so as to include the Polycystina, Thallassicollidæ and Acanthometra. To these three groups he applies the collective designation of Rhizopoda radiolaria. Lachmann, and Claparède,^[4] the latest systematic writers on this subject, have, with some few restrictions, adopted the views of Müller, and arranged the Rhizopoda as follows:—

RHIZOPODA.
						Orders.		Families.
No calcareous shell, with numerous, porous, chambers.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	Pseudopodia rarely becoming fused with one another.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \\\ \\\ \\\ \\\ \\\ \ \end{matrix}}\right.}$	No siliceous spicules or yellow cells.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	PROTEINA.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	1. Amæbina.
								2. Actinophryria.
				No siliceous spicules or yellow cells.	$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	ECHINOCYSTIDA.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	1. Acanthometrina.
								2. Thalassicollina.
								3. Polycistina.
		Pseudopodia forming very numerous, confused, agglutinations.			$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	GROMIDA.		1. Gromida.
A shell, usually calcareous, most frequently divided into several chambers, each of which, though entire, has its walls pierced with very many pores.					$\scriptstyle {\left.{\begin{matrix}\ \\\\\ \ \end{matrix}}\right\}\,}$	FORAMINIFERA.	$\scriptstyle {\left\{{\begin{matrix}\ \\\\\ \ \end{matrix}}\right.}$	1. Monothalmia.
								2. Polythlamia.

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1. Hartig, in "Quart. Jour. Micr. Sci.," 1855, p. 51; Carter, in "Ann. Nat. Hist.," 1857, p. 259; A. de Bary, in Siebold und Kölliker's "Zeitschrift," 1859, p. 88; Dickson, in "Quart. Jour Micr. Sci.," 1860, p. 7.
2. Huxley, "Lectures on Gen. Nat. Hist.," in "Med. Times and Gaz.," May 24, 1856, p 507.
3. Müller's Archiv., 1858, p. 104.
4. Etudes sur les Infusoires et Rhizopodes, 2me livraison, p. 434.