rule, at regular distances from one another. These rapidly develop
themselves into well-defined spherical air vesicles, and come
presently to fill a considerable part of the hollow of the shell,
thereby driving part of the protoplasm outside it. After from five
to twenty minutes, the specific gravity of the arcella is so much
lessened that it is lifted by the water with its pseudopodia, and
brought up against the upper surface of the water-drop, on which it
is able to travel. In from five to ten minutes the vesicles will now
disappear, the last small point vanishing with a jerk. If, however,
the creature has been accidentally turned over during its journey,
and reaches the top of the water-drop with its back uppermost, the
vesicles will continue growing only on one side, while they diminish
on the other; by this means the shell is brought first into an
oblique and then into a vertical position, until one of the
pseudopodia obtains a footing and the whole turns over. From the
moment the animal has obtained foothold, the bladders become
immediately smaller, and after they have disappeared the experiment
may be repeated at pleasure.

The positions of the protoplasm which the vesicles fashion change
continually; only the grainless protoplasm of the pseudopodia
develops no air. After long and fruitless efforts a manifest fatigue
sets in; the animal gives up the attempt for a time, and resumes it
after an interval of repose.

Engelmann, the discoverer of these phenomena, says (Pfluger's Archiv
fur Physologie, Bd. II.): "The changes in volume in all the vesicles
of the same animal are for the most part synchronous, effected in the
same manner, and of like size. There are, however, not a few
exceptions; it often happens that some of them increase or diminish