as Infusoria flagellata. 135 
have a definite oral aperture, we must, if on no other grounds, 
conclude that it also possesses a true mouth. 
Still there would appear to be some who doubt whether, after 
all, the Sponges are really animals instead of plants, and more- 
over seem to insist that they are neither the one nor the other, 
but form with other Infusorians (such as Volvox, Gonium, 
Pandorina, Euglena, and other Conferva-like bodies) a group 
by themselves, standing intermediate to, and partaking of the 
nature of, both animals and plants. This is the group which 
has been called Phytozoa, ¢. ¢. plant-animals. 
In the midst of this halting decision, I have been for some 
years past working upon a class of Infusoria the knowledge of 
whose structure fully prepared me not only to recognize the 
animal nature of the Sponge, but also enabled me to determine 
to what group of Infusoria it belongs. Such a decision, there- 
fore, does not leave any trace of doubt in my mind as to the 
strictly animal nature of the Sponges. The whole question in 
dispute hinges upon the determination as to the animal or 
vegetable nature of the Monad-like or so-called Flagellate 
Infusoria. And here, again, I would say that it has fallen to 
my lot to decide, for the first time, that one of the smallest 
of the known Infusoria, the Monad (Monas termo, Ehr. ?) is 
an animal. If, now, we can prove this point, the way is per- 
fectly clear through the intermediate forms which lie between 
the Rcsiad and the Sponge. 
Commencing, then, with what I believe to be the Monas 
termo of Ehrenberg, I shall proceed to describe in detail a 
series of forms (several of which are new, both generically and 
specifically) which stand in the closest relationship among the 
lowest embodiments of infusorial life, embracing among them, 
as I hope to show, the true ciliated Sponges, and which, not- 
withstanding, lead in unobstructed although varied courses * 
- the more elevated kinds of Protozoa, the true Infusoria 
ciliata. 
§ 1. Monas termo, Ehr. PI. V. figs. 1, 2, 3, 4. 
_ Upon aslight acquaintance with this infusorian, one would be 
ey ncuned to identify it with the younger stages of Antho- 
physa Miilleri, Bory (fig. 49); but a more searching investiga- 
tion reveals such a number of characters in each which are not 
to be found in the other, that one need not have any hesitation 
whatever in setting them down as totally diverse organisms. 
In fact Monas belongs to the uniciliate Flagellata, whilst the 
other genus just mentioned is a biciliate heteronematous form. 
Monas lives in two diverse conditions, of which one is a 
* See the preliminary remarks upon Anthophysa, § 11. 
