Diagnosis of the Discocephalina, Its families and genera
Order Hypotrichida Stein, 1859
Suborder Discocephalina (n. subord.)
Diagnosis; Thigmotactic, multimacronucleate, obligatory psammolittoral forms with marked cephalization (lacking in Erionella); somatic ciliature includes rows of long (7 um) dorsal bristles and left marginal cirri divided into anterior and posterolateral groups; right marginal cirri may be absent; a median row (reduced in some species) of midfrontal cirri differentiate, during morphogenesis, from a longitudinal series of oblique streaks.
Family Discocephalidae Jankowski, 1979
Diagnosis; Strongly cephalized hypotrichs ranging from elongate to oval in cell shape; in addition to varying numbers of midfrontal cirri, 7 other frontal derivatives may be present: a short series of right frontal cirri, anterior, malar, migratory and accessory transverse cirri plus transverse cirri arranged in a U-shaped group; both a paroral and peristomial cirrus differentiate from the paroral streak; anterior transverse microtubular bundles unite to form posterior cytoskeletal trunk.
Four genera; Type Discocephalus Hemprich and Ehrenberg, 1831.
Genus Discocephalus Hemprich and Ehrenberg, 1831
Diagnosis; Ovoid cell shape and rigid pellicle; midfrontal cirri and anterior left marginal cirral row reduced; right marginal cirri absent, extensive cirral hypertrophy; highly complex cytoskeleton; membrane-bound microtubular protrusions on cephalized region. Peristomial lobe prominent.
Type species; D. rotatorius Hemprich and Ehrenberg 1831:
1. D. rotatorius Hemprich and Ehrenberg, 1831
2. D. ehrenbergi Dragesco, 1960
Genus Marginotricha Jankowski, 1978
Diagnosis; Complete right and left marginal cirral rows; midfrontal series complete but shifted to cell's right; 3 larger frontals plus 1 paroral cirrus extend from the cephalized region; AZM extends only to anterior of cell; right frontal series absent.
One species; Type - M. grandis (Dragesco, 1954) Jankowski, 1978.
Genus Prodiscocephalus Jankowski, 1979
Diagnosis; Reduced right and left marginal cirral rows; 6 frontals, 1 paroral cirrus extend from cephalized region; AZM extends only to anterior of cell; right frontal series absent; midfrontal series reduced.
One species; Type - P. minumus (Dragesco, 1968) Jankowski, 1979.
Genus Psammocephalus n. gen.
Diagnosis; Elongate cell shape; pellicle either rigid (e.g. P. borrori) or supple (e.g. P. faurei); complete rows of midfrontal and right and left marginal cirri; peristomial lobe with peristomial cirrus prominent; AZM extends posteriorly along right lateral border of cell.
Four species: Type P. borrori n. sp.:
1. P. borrori n. sp.
2. P. dragescoi n. sp.
3. P. faurei (Dragesco, 1965) n. comb.
4. P. lithophora (Faure-Fremiet, 1954) n. comb.
Family Erionellidae n. fam.
Diagnosis; Cephalization slight; buccal cavity expansive; complete midfrontal series subtended by a J-shaped group of transverse cirri; separate anterior transverse cirral microtubular bundles; 2 right marginal rows, 1 left marginal row.
One genus; Type - E. macrostomum (Dragesco, 1963) Jankowski, 1978
Genus Erionella Jankowski, 1978
Diagnosis; Same as above.
In 1979 Jankowski proposed the superfamily Discocephaloidea with 2 families, the Amphisiellidae and the Discocephalidae. According to Jankowski, the Amphisiellidae included A. lithophora Faure-Fremiet, 1954 and A. faurei Dragesco, 1963. He divided the Discocephalidae into 2 subfamilies: the Discocephalinae, including D. ehrenbergi, D. rotatorius, Erionella macrostoma (syn. Keronopsis macrostoma Dragesco, 1963) and Stenotricha arenicolus (syn. Strongylidium arenicolus Dragesco, 1953), and the Marginotrichinae, including Marginotricha grandis (syn. Discocephalus grandis Dragesco, 1954) and Prodiscocephalus minimus (syn. Discocephalus minimus Dragesco, 1968). Because of significant morphological and morphogenetic differences, P. faurei and P. lithophora clearly do not belong in the same genus as A. marioni. A. marioni is the type species for the genus Amphisiella Gourret and Roeser, 1887. The names of families are derived from the stem of their type genera; under the Law of Priority, when a family is split, the new subgroups are to be assigned the oldest familial name available, but "the fragment containing the type genus of the original single family must retain the original familial name" (Corliss 1962). Therefore, the Amphisiellidae, as proposed by Jankowski, must be rejected. Because of developmental and morphological homologies, I consider P. faurei and P. lithophora as members of the genus Psammocephalus. Stenotricha (syn. Strongylidium arenicolus Dragesco, 1953), represented by a single species description, bears little similarity with Discocephalus. Until new information deems otherwise, I consider its systematic position uncertain and its placement with Discocephalus-like hypotrichs unjustified. Erionella (syn. Keronopsis macrostoma Dragesco, 1963) appears to have such Discocephalus-like characters as 2 differentiated groups of left marginal cirri, long dorsal bristles, a row of median (midfrontal?) cirri, and multimacronuclearity; it lacks, however, a marked cephalization and has a different arrangement of transverse cirri and associated cytoskeletal components. Although morphogenetic data is needed o verify apparent homologies. I provisionally include Erionella in the Discocephalina; it is sufficiently divergent from the Discocephalids, however, to warrant separation at the familial level. The species composing the genus Psammocephalus form a continuous morphological series (phenocline) that culminates in the genus Discocephalus. I consider Psammocephalus, along with the more divergent Marginotricha and Prodiscocephalus, as belonging to the Discocephalidae.