During the past few decades the protozoan genus Tetrahymena (Furgason, 1940) and related members of the family Tetrahymenidae (Corliss, 1952) have perhaps received more notice in the scientific literature than any other single protozoan group. Nine genera belonging to this family and thirteen species of Tetrahymena are listed by Corliss (1959, 1961). Classification of genera comprising the family, and of species within the genera, is based on morphological features revealed by the silver line technique, such as ciliary meridians (type, number, and anterior convergence patterns), the contractile vacuole pores (number and relative position with respect to the ciliary meridians), and the buccal apparatus (with its three ciliary membranelles, an undulating membrane, and cytopharynx). Hundreds of strains of T. pyriformis have been isolated and established in clonal culture, not to mention others still unidentified and the amicronucleated strains that have been used in many laboratories for years. Morphological features such as those mentioned above have been noted in many publications. In one recent comparative study of 36 strains the investigators (Loefer et al., 1960) concluded that "the morphological variables noted fall within a limited range" and the data furnished little valid justification for the retention of separate strain designations. If valid morphological criteria cannot be found for separation of these strains of T. pyriformis, the question arises whether certain physiological or biochemical characters might be of taxonomic utility. Separation of protozoan subgroups or populations of a species into varieties (or syngens) has, of course, been accomplished, using the physiological criterion of mating type affinity. It shoulld be noted that the term "variety," followed by a numeral, as used to designate protozoan populations, has a physiological basis and connotation different from ordinary taxonomic usage. Twelve varieties of T. pyriformis are known to date (Elliott et al., 1962), and each may include a number of mating types. We have been conducting comparative investigations in two areas of protozoology: (a) on antigenic similarities and differences in the Tetrahymenidae as revealed by the immobilization reaction, and (b) on the free amino acid patterns of different genera and species of the Tetrahymenidae. Inasmuch as taxonomic criteria have been suspected to exist in these fields, it seemed worthwhile to review our findings for whatever light they might shed on the taxonomy of these Protozoa. A study (Loefer et al., 1958) of the serological cross reactions of 31 strains of T. pyrifo'rmis, cultured axenically in identical media at 26?C, showed that these strains could be allocated to fourteen serological categories. Additional observations indicated that a clone of a given strain, if maintained in culture at a lower temperature, e.g., at 10?C, oftentimes transformed to a serotype different from that exhibited in the first instance. Temperature-induced transformations were subsequently shown for several strains of variety 1 (Margolin et al., 1959). In more extended studies (Loefer and Owen, 1961) on seven wild and 71 derived strains of variety 1, at least six antigenic specificities could be demonstrated. Additional data (Loefer, unpublished) show that most of the strains which
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The significant cytostomal pattern of the "Glaucoma-Colpidium group" and a proposed new genus and species, Tetrahymena geleii.
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