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con- sidered without the prefix; many structures and organelles that are (micro)fibrillar in composition are composed of non-hollow filaments 4\u201310 nm in diameter; the term is most frequently used in its adjectival form; fibrillar or microfibrillar constituents may include such prominent and organized structures as the kinetodesma , myoneme , and spasmoneme , and perhaps the karyophore and the filamentous annu- lus ; \u201cfilamentous\u201d may be used to describe the very same organelles; see Microfilament . Microfilament : generalized term, perhaps better considered without the prefix; the finer or finest composition ( ca . 5 nm in diameter) of a number of important organelles appears to be microfilamen- tous in nature, often densely so and with or without nodes; microfilamentous structures may include the epiplasm , the filamentous reticulum , and the infraciliary lattice ; if this is the ultimate or lowest macromolecular level of organization, then there should be a distinction, even in very generalized usage, between this term and microfibrils , but this has not always been the case in the literature \u2013 the terms have been used interchangeably by some workers; see Microfibril . Microgament : see Gamont . Micronucleus (pl. Micronuclei ): so-called gen- erative nucleus, typically much smaller than the macronucleus ; may be multiple, generally spheri- cal or ovoid in shape, and typically diploid in its genomic content; without nucleoli and typically showing no transcriptional activity; its nuclear envelope with pores in some species, without them in others; divides mitotically or meiotically, play- ing a major role in sexual phenomena, such as autogamy and conjugation ; absent in amicronucleate strains or races (Mi, Figs. 2.9Af, 2.12). Microphagous : feeding on small or very small particles of food; a generalized term embracing especially bacterivorous and sometimes algivorous feeding; to be contrasted with carnivorous, histopha- gous, saprozoic, and especially macrophagous . Micropyle : differentiated pore in the wall of a resting cyst through which the ciliate emerges on excystment; the pore canal is sealed by cyst wall material; found in some spirotrichs and colpodeans (Mpy, Fig. 2.9Ag). Microstome : a stage in the polymorphic life cycle in which the oral apparatus undergoes morphogenesis to become reduced in size and capable of ingesting only small prey items, typically bacteria; see Macrostome (Fig. 2.4A). 38 2. Glossary of Terms and Concepts Useful in Ciliate Systematics Microstome-Macrostome Transformation : see Stomatogenesis . Microtoxicyst : used as a synonym of haptocyst , but might also refer to some other minute toxicyst . Microtubular Ribbons : a set of microtubules aligned laterally to form a flat \u201cribbon-like\u201d struc- ture; the most striking microtubular ribbons include the transverse and postciliary microtubules , and the microtubular arrays in the suctorial tentacle (Figs. 2.1, 2.2, 2.10). Microtubule : hollow, cylindrical structure of indeterminate length, ca . 20\u201325 nm in diameter, composed of subunits of tubulin ; rigid, often cross- linked with others to form a microtubular ribbon or nematodesma ; microtubules in the cytoplasm are typically associated with the kinetosome (Figs. 2.1, 2.2, 2.10). Microzooid : see Zooid . Migratory Form : see Larval Form . Missile-like Body : see Haptocyst . Mitochondrion (pl. Mitochondria ): generally con- spicuous organelles in the cytoplasm, composed of a complex membrane system with the inner membrane appearing to form cristae of several types, usually tubular in ciliates, and indispensably functioning as the \u201cpowerhouse\u201d of the cell; in some ciliates, arranged in specific (often linear) patterns or formations; in scuticociliates , apparently fused (?) in a single interconnected \u201ccompound\u201d mitochondrion, a giant chondriome located immediately under the pellicular alveoli; in some ciliates, independently transformed to a hydrogenosome (e.g., armophor- eans , litostomes , plagiopyleans ). Mixokinetal : stomatogenesis in which both paren- tal somatic kineties and parental oral structures are simultaneously involved in development of the opisthe\u2019s oral anlage ; found in nassophoreans , apostomes , and the spirotrich Protocruzia . Mixotrophic : capable of using two or more modes of nutrition (e.g., autotrophic and heterotrophic ). Monogemmic : production of a single bud (at a time); a mode of fission . Monokinetid : a kinetid composed of one kineto- some and its fibrillar associates; see Dikinetid , Dyad , and Polykinetid (Figs. 2.1E, 2.2). Monoparakinetal : parakinetal stomatogenesis in which only one somatic kinety is involved in for- mation of the oral anlage ; found in tetrahymenids (Fig. 2.11Dd). Monophyletic : condition of a taxon being com- prised of a common ancestor and descendants all presumed to be derived from this common ances- tor; established by the cladistic approach through the sharing of apomorphic or derived characters ; see Clade , Paraphyletic , and Polyphyletic . Monostomy : condition of having but one cytostome . Monotelokinetal : telokinetal stomatogenesis in which the oral anlage is derived by proliferation of kinetosomes in the somatic portion of oral kineties ; found in pleurostome haptorians . Monotomic : division of a single individual into but two filial products ; the mode of fission typical of most ciliates. Monotypic : a taxonomic group having only one included nominal taxon; for example, a monotypic genus includes only one species. Monoxenic Culture : literally \u201cone stranger\u201d culture; laboratory growth of two kinds or species of living organisms with no others present; for example, a ciliate plus one \u201cstranger\u201d \u2212 a bacterium, an alga, a yeast, or another ciliate species; the second organism is typically present in the medium to serve as food for the ciliate of interest, which is usually being studied biochemically or ecologically. Morphogenesis : coming-into-being of characteristic and specific form; the transformation involved in growth and differentiation or ontogeny , resulting in reproduction of the preexisting form, with the same patterned array of cytoarchitectural substruc- tures; morphogenetic movements are involved in the process of fission , but also in cystation , conju- gation , regeneration , and particularly in stomato- genesis ; the consistent patterns of such dynamic ontogenetic phenomena may be of considerable value in both phylogenetic and comparative taxo- nomic work; see also Biogenetic Law . Morphological Species : an assemblage of popula- tions of organisms that share a strong and stable morphological similarity; often assumed by taxono- mists to represent a biological species , but likely to represent a number of different biological entities. Glossary 39 Morphospecies : see Morphological Species . Motorium : see Neuromotorium . Mouth : of value only as a very general term, used in reference to the oral region of any mouth-bearing ciliate; the \u201ctrue\u201d mouth of a ciliate should be called the cytostome . Mucigenic Body : see Mucocyst . Mucocyst : cortical, membrane-bound, saccular or rod-shaped extrusome with a paracrystalline struc- ture; dischargeable as an amorphous, mucus-like mass through an opening in the pellicle; probably involved in cyst formation, among other possible functions; occurs in regular, longitudinal, interki- netal rows in many ciliates; formerly known as a protrichocyst (especially), a mucous trichocyst, or a mucigenic body; an ampullocyst has been consid- ered a special type of mucocyst (Fig. 2.9Cb). Mucous Trichocyst : not a trichocyst ; see Mucocyst . Müller\u2019s Vesicle : small vacuole containing mineral concretions, and functioning as a gravity receptor; found in karyorelicteans