This division includes all animalcules having an internal digestive canal provided with a mouth and anal opening.
4th Section. ANOPISTHIA.
Mouth and anus contiguous.
5th Section. EN A NTIOTR ETA .
Mouth and anus terminal and opposite ; re production by transverse division.
6th Section. ALLOTRETA.
Mouth and anus terminal and opposite, as in the last section ; reproduction by longitudi nal and transverse division.
7th Section. KATOTRETA.
Mouth and anus not terminal ; reproduction as in last section.
Taking the above classification for our gui dance, we must now proceed to investigate more minutely the organization of the strange animals included in this extensive series.
Locomotion.—Although no special locomo tive apparatus has as yet been discovered in the family of Monads, this perhaps depends rather upon our deficient means of investigation than upon their absence. Attentive observation shews that every true Monad is furnished with a minute filiform proboscis, (I, 2, 3, fig. 1,) which, as it constantly exhibits an un dulatory or vibratory motion, has been mis taken by some observers for a ciliary apparatus. Sornetimes two of these organs are present, but this cannot be regarded as an essential charac teristic feature, seeing that during the process of spontaneous fissure an animalcule which previously had only one proboscis, becomes furnished with two preparatory to its separation into two individuals. In some species, how ever, two are constantly present. These pro boscides may possibly discharge a double func tion, and perform the duty both of locomotive and of prehensile organs with which to collect nourishment.
In the Cryptomonadinid likewise one or two filiform proboscides, similar to the above, seem to be the locomotive organs ; and the vibratile apparatus that serves for the move ments of the Volvoces is entirely composed of similar structures belonging to the individual animalcules that constitute the compound bo dies of these wonderful beings.
Amongst the Vibrionidw the locomotion is of a very different character. In the true Vibrios it is performed by a kind of meandering or undulating movement, the fibre-like com pound body of the animal bending and straight ening itself alternately, the cause of which seems to depend upon a stronger binding together and subsequent relaxation of the individual animalcules, so that these seem to displace one another. In Bacterium the contraction is weaker, so that no undulating movement is produced, although the creature swims straight forward.
In the family Closterina (6, 7,.fig. 5) the lo comotive organs consist of numerous short, soft, conical papillee, situated near the openings of the shell at the two opposite extremities of the animal ; they are placed upon the inner side, and can be protruded but a very little way from the shell.
In the family Amoeba no special locomo tive organs are met with. The round, gelati nous, and highly contractile bodies of these creatures have the capability of thrusting out at will foot-like processes from any part of their body, by the assistance of which they manage to move about. A similar mode of progression is met with in the Arcellinidm. In all the higher forms of Polygastric Infusoria locomo tion is effected by means of cilia variously distributed over different parts of the body, but their position in different genera will be described when speaking of the external forms of the different families.
These cilia are described by Ehrenberg to be minute hairs arising from a thick bulbous basis, upon which they execute a rotatory mo tion, some of them being continuous with their basis, while others are only articulated there unto ; of these the former kind exists in Stylo nychia mytilus, and the latter in Paramecium aurelia.