Order Heterotrichida, Odontostomatida, Oligotrichida, Hypotrichida
Representative cysts are drawn in Fig. 1-17 for each of the orders except for the odontostomatids, cysts of which are presently unknown. Immediately apparent is the strong similarity between the cysts of heterotrichs, tintinnids, an oligotrich and the possibly related cysts in the incertae sedis group, papuliferes (Meunier 1910; Reid and John 1978, 1981). All, with the exception of the supposed cyst of Phacodinium (Penard 1922), are characterized by distinctive oval to spherical resting cysts with a protrusion at one end marking the site of the excystment aperture, which is closed by a hyaline plug (Paranjape 1980; Reid and John 1978). In the papuliferes this opening may at times be covered by a reticulated 'bud-like' extension, which may be equivalent to the 'frothy plug' of Strombidium (Faure-Fremiet 1948). Flask-shaped cysts are now known for at least eight genera of tintinnids: Acanthostomella (Paranjape 1908), Eutintinnus (Heinbokel, pers. comm.), Favella (Reid and John 1978), Helicostomella (Reid and John 1978), Leprotintinnus (Reid and John 1978), Metacylis (Reid and John, unpub.), Parundella (Reid and John 1978), and Tintinnopsis (Reid and John 1978); one genus of oligotrich: Strombidium (Faure-Fremiet 1948), and five genera of heterotrichs: Blepharisma (Giese 1973; Repak 1968), Fabrea (Bussers 1976; Kirby 1934), Stentor (Stein 1867), Nyctotherus (Lucas 1928), and Paraclevelandia (Kidder 1937) but are not known in other classes of the Ciliophora. The cysts range in size from approximately 30 to 140 um and have smooth walls that appear to be made up of two main layers and a third fragile, at times membranous, outer layer. The majority of forms are resistant to immersion in strong acids and alkalins (Bussers 1976; Paranjape 1980; Reid and John 1978, 1981). Some cysts attributable to the papuliferes may be ornamented by solid spines or membranous folds. Cytological changes during encystment have only been studied for a small number of species; dedifferentiation of ciliary and oral structures appears to take place (Bussers 1976; Repak 1968). To our knowledge, cysts have not so far been recognized in the heterotrich suborders: Armorphorina, Plagiotomina, and Licnophorina. Possible resting cysts in the suborder Coliphorina (elongate with a membranous outer wall) have only been recorded once (Hadzi 1951); the type of aperture and method of excystment is not known. In marked contrast to the cysts of the previous groups, the Hypotrichida have simple spherical cysts, without any projection or pylome, and dehisce via a split in the cyst wall. An exception is the pear-shaped cyst of the loricate genus Chaetospira (Froud 1949), which has an unknown method of excystment. Cyst formation is common in the hypotrichs and is known for at least fifteen genera in eight of the eleven families recognized in the classification of Levine et al. (see ref. ID; 923): Aspidisca (Curds, pers. comm.), Chaetospira (Froud 1949), Diophrys (Walker and Maugel 1980), Euplotes (Faure-Fremiet, Gauchery and Tuffrau 1954; Garnjobst 1937), Gastrostyla (Walker, Maugel and Goode 1980; Weyer 1930), Histriculus (Curds 1966), Keronopsis (Penard 1922), Onychodromus (Jareno and Tuffrau 1979), Oxytricha (Grimes 1973), Paraholosticha (Tuffrau and Fryd-Versavel 1977), Pleurotricha (Ilowaisky 1926; Jeffries 1956), Strongylidium (Pack 1919), Stylonychia (Ilowaisky 1926; Walker, Maugel, and Goode 1975), Stylonethes (Garnjobst 1937), Urostyla (Tittler 1935). Cysts of these genera are from 15 to 70 um in diameter. Ultrastructural studies have shown that the cyst wall may be composed of up to four layer (Walker, Maugel and Goode 1980). The outer layer is smooth or may be folded to form ridges or 'spine-like' projections which are merely extensions of the outer wall and not rigid structures. Little is known regarding the chemical characteristics of the cyst wall; cysts of Euplotes are resistant to acids and alkalins (Faure-Fremiet, Gauchery and Tuffrau 1954). Hypotrichs have in the past been considered to represent the peak of ciliate development or morphological differentiation and to comprise a closely knit assemblage of species (Corliss 1979). This traditional view has recently been placed in doubt (Small and Lynn 1981), with the suggestion that Euplotes and related genera should be distinguished from the hypotrichs sensu stricto. Evidence from cytological and fine structural studies of cysts supports this division; striking constrasts are evident between cysts of the euplotid ciliates Diophrys and Euplotes and other hypotrichs such as Gastrostyla and Stylonethes (Faure-Fremiet, Gauchery and Tuffrau 1954; Garnjobst 1937; Walker and Maugel 1980; Walker, Maugel and Goode 1980). The cysts of Diophrys and Gastrostyla are distiguishable on a number of criteria, the most characteristic of which is whether the kinetosomes are resorbed or maintained within the resting cysts; in hypotrichs sensu stricto mature cysts are completely dedifferentiated and contain no kinetosomes or cilia (Walker and Maugel 1980).
Discussion; Major differences in shape, excystment aperture, walls structure and cytology are evident between the cysts of both the heterotrichs and oligotrichs compared to the hypotrichs, and in two separate groups within the hypotrichs. These sharp contrasts, in what appear to be highly conservative morphological traits, imply a different phylogenetic origin for each of these cyst groups and suggest that the Polyhymenophorea as construed by Levine et al. (see ref. ID; 923) is polyphyletic. In the classification of Small & Lynn (1981), the Polyhymenophoria has been reduced to the level of a subclass and 'tentatively' attributed, together with the Hypostomia, to a new class the Nassophorea. Only the heterotrichs and odontostomes are retained within the Spirotrichea, now raised to the level of a class. The hypotrichs and oligotrichs are included together in the Polyhemenophoria, but it was pointed out that they may well be divided in the future and in part be allocated to the same subphylum as the Spirotrichea. This survey of cyst forms has served to confirm and strengthen most of the observations of Small & Lynn. We propose that, within the classification, the Order Oligotrichida should be included in the Spirotrichea with the Heterotrichida, and that the Hypotrichida should be divided with the creation of a new taxonomic group (order or suborder) to include Euplotes and related genera. Ciliate cysts have been largely ignored in the past and have only been described for a small number of species, despite the fact that their formation is a common feature of the phylum (Corliss 1979; Corliss and Esser 1974). Encystment appears to be a primitive feature of the class Polyhymenophorea and possibly also of the class Vestibulifera. Hashimoto (1966) has demonstrated, for instance, that the kineties within the cysts of Colpodidae assume what he considers to be a 'primitive' pattern during reorganization on excystment. Maintenance of conservative morphological characteristics in cysts may be a consequence of a slowing down of selection pressures within cysts because of their reduced metabolism and protection from the variable environmental stresses to which the trophic stage is subject. This work has served to emphasize the possible importance of cysts as a new tool for determinating phylogenetic relationships and demonstrates the need for a greater emphasis to be placed on the study of encystment in the biology of ciliates.