as Infusoria flagellata. 135 



have a delinite oral aperture, we must, if on no other grounds, 

 conckide 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 Avith other Infusorians (such as Volvox, Goni'um, 

 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, ?'. e. 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 ternio, 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 Monad 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 * 

 to the more elevated kinds of Protozoa, the true Infusoria 

 ciliata. 



§ 1. Monas termo, Ehr. PI. V. figs. 1, 2, 3, 4. 



Upon a slight acquaintance with this infusorian, one would be 

 strongly inclined to identify it with the younger stages ofAntho- 

 j>hysa Mulleri, 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. 



