in some mysterious way an organic connexion with its companions. It is not easy to understand how a number of distinct beings can move in such perfect unison as to be frequently mistaken for a single animalcule. Yet so it is; this group of monads rolls round and round the drop of water, with the peculiar revolving or spinning movement which has given rise to its distinctive appellation of volvox, just as if it were a simple being. Six or eight young volvoces may generally be seen through the transparent envelope, from which they make their escape when sufficiently developed to become the envelopes of new broods.
The Rotifera form a class even more interesting than the monads. The animals of this class have usually an elongated form, and are perfectly symmetrical on the two sides. Near the mouth we observe one or two rows of delicate cilia, which are frequently arranged in a circular manner; and when they are in motion, an appearance of revolving wheels is produced, from which the class derives its appellation. The common wheel animalcule was long a puzzle to philosophers, who were forced to invent many marvellous hypotheses to explain the motion of the pair of paddle-wheels with which this little creature is furnished. We must not always believe our own eyes—for the two little wheels on the anterior part of the body of this rotifer, which seem to be always turning round on their axes, are really stationary. The motion is now
