The spectacle, though always surprising, was not new to me. So my reply to the question put to me was, that these organisms were what biologists call _Monads_, and though they might be animals, it was also possible that they might, like the _Bacteria_, be plants. My friend received my verdict with an expression which showed a sad want of respect for authority. He would as soon believe that a sheep was a plant. Naturally piqued by this want of faith, I have thought a good deal over the matter; and as I still rest in the lame conclusion I originally expressed, and must even now confess that I cannot certainly say whether this creature is an animal or a plant, I think it may be well to state the grounds of my hesitation at length. But, in the first place, in order that I may conveniently distinguish this “Monad” from the multitude of other things which go by the same designation, I must give it a name of its own. I think (though, for reasons which need not be stated at present, I am not quite sure) that it is identical with the species _Monas lens_, as defined by the eminent French microscopist Dujardin, though his magnifying power was probably insufficient to enable him to see that it is curiously like a much larger form of monad which he has named _Heteromita_. I shall, therefore, call it not _Monas_, but _Heteromita lens_. I have been unable to devote to my _Heteromita_ the prolonged study needful to work out its whole history, which would involve weeks, or it may be months, of unremitting attention. But I the less regret this circumstance, as some remarkable observations recently published by Messrs. Dallinger and Drysdale[34] on certain Monads, relate, in part, to a form so similar to my _Heteromita lens_, that the history of the one may be used to illustrate that of the other. These most patient and painstaking observers, who employed the highest attainable powers of the microscope and, relieving one another, kept watch day and night over the same individual monads, have been enabled to trace out the whole history of their _Heteromita_; which they found in infusions of the heads of fishes of the Cod tribe. Of the four monads described and figured by these investigators, one, as I have said, very closely resembles _Heteromita lens_ in every particular, except that it has a separately distinguishable central particle or “nucleus,” which is not certainly to be made out in _Heteromita lens_; and that nothing is said by Messrs. Dallinger and Drysdale of the existence of a contractile vacuole in this monad, though they describe it in another.