pecies _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[5] 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.

[Footnote 5: "Researches in the Life-history of a Cercomonad: a Lesson in
Biogenesis"; and "Further Researches in the Life-history of the Monads,"
--_Monthly Microscopical Journal_, 1873.]

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.

Their _Heteromita_, however, multiplied rapidly by fission. Sometimes a
transverse constriction appeared; the hinder half developed a new cilium,
and the hinder cilium gradually split from its base to its free end,
until it was divided into two; a process which, considering the fact that
this fine filament cannot be much more than 1/100000 of an inch in
diameter, is wonderful enough. The constriction of the body extended
inwards until the two portions were united by a narrow isthmus; finally,
they separated and each swam away by itself, a complete _Heteromita_,
provided with its two cilia. Sometimes the constriction took a
longitudinal direction, with the same ultimate result. In each case the
process occupied not