The ciliates of domestic animals all belong to the class Ciliasida. The nuclei of this group are unique in the animal kingdom. Every individual (except in a few amicronucleate strains) has a micronucleus which contains a normal set of chromosomes, and a macronucleus which contains an indeterminately large number of sets and is actually n-ploid rather than polyploid. The micronucleus is active in reproduction, while the macronucleus has to do with the vegetative functions of the organism.
The ciliates have either simple cilia or compound ciliary cirri or membranelles in at least one stage of their life cycle. They also have an infraciliature in the cortex beneath the pellicle, composed of the ciliary basal granules (kinetosomes) and associated fibrils (kinetodesmata). The infraciliature can be stained with silver, forming the so-called silver-line system. Reproduction is by transverse binary fission, in contrast to the longitudinal fission seen in the flagellates. True sexual reproduction, in which gametes fuse to form a zygote, is absent, but conjugation, in which there is an exchange of micronuclear material between two individuals, may be present.
The great majority of ciliates are free-living, but a number are parasitic. Their classification has recently undergone considerable overhauling, and they are now arranged in 26 orders and suborders belonging to 2 subclasses (Corliss, 1956, 1957, 1959). This classification is based on recent work by the French school, and particularly by Faure-Fremiet, on the silver-line system, and is more natural than the earlier one. Terms used in describing the ciliates are defined by Corliss (1957). Corliss (1961) has reviewed the whole group.
The characteristics of the taxa found in domestic animals have been given on pp. 34-38. In the subclass Holotrichasina, the body ciliature is typically uniform and simple. Buccal ciliature (an adoral zone of membranelles) is usually absent or inconspicuous. This subclass contains 4 orders of veterinary interest. In the order Gymnostomorida ("naked mouth"), the cytostome opens directly at the surface or else into a slight depression which has no oral ciliature. This order includes 2 families of which members occur in the large intestine of equids or ruminants or in the rumen and reticulum of ruminants.
In the order Suctoriorida only the young have cilia, while the adults have tentacles. All members of this order are free-living except for one genus which occurs in the large intestine of equids.
In the order Trichostomorida ("hair mouth"), the cytostome is usually at the base of a well-defined oral pit or vestibulum, which in turn may sometimes be preceded by an oral groove. The vestibular wall bears I or more dense fields of adoral (vestibular) cilia. The great majority of trichostomes are free-living, but there are 5 families which contain parasites of domestic animals.
In the order Hymenostomorida ("membrane mouth"), the adoral cilia are fused in membranes, the number, size and arrangement of which vary in different genera. The free-living genera Paramecium and Tetrahymena belong to this order; the latter is occasionally parasitic. The most important parasite in the order is Ichthyophthirius, which is often a serious pathogen of aquarium fish, causing a disease known as "ick".
In the subclass Spirotrichasina, the buccal ciliature, and especially the multipartite adoral zone of membranelles, is conspicuously developed. The body ciliature is typically sparse, and all the simple cilia may even be replaced by cirri; in one order, the Heterotrichorida, however, the body ciliation is usually complete.
This subclass contains 2 orders of veterinary interest. In the order Heterotrichorida the somatic ciliation is usually complete. One genus in this order, Nyctotherus, occurs in amphibia and various invertebrates, but has also been found in the feces of ruminants. The order Entodiniorida contains a group of remarkably bizarre genera which occur in ruminants and equids. In this order, ciliation may be limited to the adoral zone or there may be 1 or more additional bands or groups of membranelles. The internal anatomy is complex, and unique "skeletal plates" may be present. There are 2 families. The Ophryoscolecidae have not more than 1 "dorsal" or "metoral" band of membranelles in addition to the adoral zone and occur in the rumen and reticulum of ruminants. The Cycloposthiidae have 2 or more bands of membranelles in addition to the adoral zone, and occur in the large intestine of equids and also of anthropoid apes.