Gymnostomes (Order Gymnostomatida)
As the name suggests the mouth is without ciliary organelles. This is a most diverse group of structurally simple ciliates. But diversity has led to considerable changes in classification in recent years. A comparison of current classifications shows that no general agreement has yet been reached. Most species in the order contain one meganucleus and one or more micronuclei. The mouth may be anterior and terminal or lateral. If the mouth is lateral then the body is produced into a thin extension to form the new anterior end. This is the basis for the classification of the suborder Rhabdophorina, in which the cytopharynx (the space immediately within the cell opening, the cytostome) is extensible. The other suborder is the Cyrtophorina which has a rigid inextensible cytopharynx.
In this group are found some remarkably voracious feeders, carnivorous in every sense of the word despite their smallish size. A simple form like Holophrya is oval and about 160 µ long. It is found on the surface of freshwater where it feeds on dinoflagellates. The remains of past meals are clearly visible in several food vacuoles in the cytoplasm. A contractile vacuole partly maintains the water balance in the cell. The Didinium is similar in shape but has much reduced ciliary fields. Only two bands are left, one around the mouth and one near the equator. This organism can ingest a whole Paramecium although the prey is several times larger than itself. The vigour with which the prey is attacked, apparently paralyzed, and then drawn into a mouth that seems far too small is fascinating to watch. Other genera include Dileptus and Lionotus which have extended trichocyst and cilia-bearing anterior processes giving a comb-like appearance to the region in front of the mouth. This shape is an adaptation to living between sand grains and environments where free swimming is impossible.
The ventrally placed mouth is believed to be of evolutionary significance in view of the fact that the majority of ciliates are like this but have additional elaborations. This is a small group. A typical genus is Chilodonella. The ventral mouth is visible from the dorsal side through the cell. The anterior process is considerably thinner than the main posterior part. A line of cilia, the dorsal brush, runs from the edge of the cell to a region over the mouth, and should not be confused with the feeding organelles in other ciliates which sweep food into the mouth.
Suctorians (Order Suctorida)
The suctorians are a compact group only recently. considered as holotrichs. The adult form is sessile and without cilia. Food is captured by long knob-ended tentacles which are capable of paralyzing prey. After capture the prey is not drawn to the body, for there is no mouth, but is sucked dry by the tentacles. Recent film studies show the steady flow of cytoplasm from an organism such as Tetrahymena into a suctorian. Reproduction is done by several methods. The commonest is internal budding. After the division of the nucleus, the anterior pellicle sinks into the cytoplasm in a circle and a young ciliated suctorian develops around the new nucleus. When mature the daughter cell leaves the parent and swims freely. Later it settles down on some object, secretes a non-contractile stalk, and loses its cilia to become a small adult. Stained preparations of suctorians show that at all stages of development the kinety system is present even when cilia are not. Typical genera are Tokophyra, Podophyra, and Acineta.
Trichostomes (Order Trichostomatida)
This group has many features in common with the gymnostomes but is distinguished from them by the presence of a vestibulum. This organ is a depression of the pellicle complete with the cilia which form a tube leading to the mouth. The cilia of the vestibulum urge food into the mouth. The cytoplasm adjoining the mouth appears to be highly vacuolated, and algae or bacteria upon which a free-living genus, Coelosomides, feeds, are digested here.
The structure of the gut parasite of amphibians and mammals, Balantidium coli, superficially resembles some hymenostomes or heterotrichs. The mouth, however, is fed by vestibular cilia which are derived from and develop in a way similar to the normal somatic cilia covering the cell. A common freshwater genus is Colpoda, characteristically kidney-shaped and about 75 µ long. The mouth is on the indented side, and the vestibular cilia, clearly visible in stained preparations, are a little shorter than the somatic cilia.