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Other writers go farther, and propose to unite the Sponges with the Rhizopoda; nor can we hesitate to admit that many facts are in favour of such a conclusion. But, on the other hand, the possession of so remarkable a morphological element as their fibre, which, even when siliceous, as in Dictyochalis, still exhibits its characteristic reticulated arrangment, must, apart from their aquiferous system, and the comparatively large size which many of them attain, be duly estimated in all attempts to lower the independent value of this group.

The Gregarinida, at first sight, appear to depart most in structure from the ordinary type of an Astomatous Protozoon. Yet even these, as Lieberkühn has shown, give rise, in the course of their development, to bodies which very closely resemble Amæbæ. Thus intimately are these humbler organisms allied to one another.

We are, therefore, disposed to conclude that the time has not yet come for adopting any very definite sub-division of the Protozoa. Those, however, who are not content with the simple enumeration of groups given above, may adopt such a provisional arrangement as the following:—

PROTOZOA.
$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad \quad } }$
ASTOMATA.		STOMATAODA.
$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad } }$		$\scriptstyle {\overbrace {\quad \quad \quad \quad \quad \quad \quad } }$
1. Rhizopoda.		1. Infusoria.
2. Spongida.		2. Noctilucida.
3. Gregarinida.

Such characters as appear to be common to all the Protozoa are, for the most part, purely negative. They do not present that differentiation into distinct layers, which, at so early a stage of development, is manifested by the members of the remaining sub-kingdoms. Those who prematurely seek for a definition of the Protozoa must remember that, in beings of the simplest plan of animal structure, the presence of positive anatomical features, similar to those which distinguish groups of a higher grade of organization, is not to be expected.

 

We look forward, then, with hope to the result of further investigations, for a solution of those difficulties which yet stand in the way of a right knowledge of the Protozoa. Nevertheless, a writer so eminent as Professor Agassiz has, in a recent work, declared that the very existence of this division, as a distinct animal sub-kingdom, must henceforth be ignored. We quote, verbatim, the passages in which he professes to have arrived at so iconoclastic a conclusion:—

"As to the Protozoa, I have little confidence in the views generally entertained respecting; their nature. Having satisfied myself that Colpoda and Paramecium are the brood of Planariæ, and Opalina that of Distoma, I see no reason why the other Infusoria, including in Ehrenberg's division Enterodela, should not also be the brood of the many lower worms, the development of which has hitherto escaped our attention. Again, a comparison of the early stages of development of the Entomostraca with Rotifera might