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net photosynthesis. The sym- bionts can be a facultative association between a Chlorella and Coleps or Holophrya (formerly Prorodon ) (Christopher & Patterson, 1983) or the Holophrya may actually sequester chloroplasts of prey (Blackbourn, Taylor, & Blackbourn, 1973). As with other mixotrophs , symbiont-bearing Coleps had growth rates higher than aposymbiotic forms when food resources were scarce (Stabell, Andersen, & Klaveness, 2002). Symbiotic bacteria have been reported in Urotricha ovata (de Puytorac & Grain, 1972). As conspicuous components of microbial food webs , it is not surprising that prostomes are them- selves eaten by other organisms. A variety of predatory ciliates, such as Dileptus , Lagynophrya , Lacrymaria , Didinium , and Favella (Jürgens, Skibbe, & Jeppesen, 1999; Seravin & Orlovskaja, 1977; Stoecker & Evans, 1985), prey upon pros- tomes , such as Coleps and Balanion . Even though Tiarina may feed upon the autotrophic dino- flagellate Dinophysis , when the dinoflagellate is heterotrophic, the table is literally turned and predator becomes prey (Hansen, 1991)! Prostomes are also eaten by the rotifers Brachionus (Mohr, Gerten, & Adrian, 2002) and Keratella (Jack & Gilbert, 1993). Microcrustacean zooplankton are also predators of prostomes : in fresh water by the cladocerans Daphnia and Bosmina (Jack & Gilbert, 1993; Wickham & Gilbert, 1991) and the copepod Cyclops (Wickham, 1995); and in marine environments by the copepod Acartia (Olsson, Granéli, Carlsson, & Abreu, 1992). Even the jel- lyfish Aurelia preferred the prostome Urotricha over dinoflagellates (Stoecker, Michaels, & Davis, 1987b). Balanion is able to avoid predation by marine copepods by escape jumping in response to fluid mechanical signals generated by the preda- tor\u2019s feeding apparatus (Jakobsen, 2001), and some Urotricha species may have a similar ability (Tamar, 1979). Finally, Hiller and Bardele (1988) make the impor- tant point that the stages in the life cycle of some prostomes, such as the histophagous Holophrya (formerly Prorodon ), can be so different in size and shape that one might consider them to be different species. Thus, careful and continuous observation is necessary to confirm that this is in fact not the case. 13.3 Somatic Structures Prostomes are generally ovoid to ovoid elongate in shape, but they can range from nanociliates (i.e., < 20 µm in body length), as some Urotricha species do, to several hundreds of micrometers in length, as some Holophrya (formerly Prorodon ) species do (Fig. 13.2). They are typically not highly contractile, although their bodies are flexible and, in histophagous and parasitic forms, able to penetrate the tissues of hosts. A few species, such as Vasicola , can form a lorica (Dragesco et al., 1974). Since the species diversity of this class is not great, there have also been relatively few studies of the ultrastructure on this group. Species in the genera Balanion , Bursellopsis , Coleps , Cryptocaryon , Holophrya Fig. 13.2. Stylized drawings of representative genera from orders in the Class PROSTOMATEA . A member of the Order Prostomatida \u2013 Apsiktrata . Members of the Order Prorodontida . Urotricha , Coleps , and Holophrya (formerly Prorodon ) 264 13. Subphylum 2. INTRAMACRONUCLEATA: Class 7. PROSTOMATEA (formerly Prorodon ), Placus , and Urotricha have been investigated. The plasma membrane of prostomes rarely exhibits a glycocalyx , although a very thin and inconsistent layer can be observed in some micro- graphs of Holophrya (formerly Prorodon ) (Hiller, 1993a). The underlying alveoli are often swollen and can be occupied partially with smaller par- ticles in Holophrya (formerly Prorodon ) (Hiller, 1993a), bacteria in Placus (Grain et al., 1979), completely with dense material in both Holophrya (de Puytorac, 1964) and Cryptocaryon (Colorni & Diamant, 1993; Matthews et al., 1993), and with calcium carbonate skeletal plates in Coleps (Fauré-Fremiet, André, & Ganier, 1968b). The fine structure of these skeletal plates differs in morphology among Coleps species (Huttenlauch, 1985). The epiplasm is conspicuous in larger forms with a fine layer observed in Holophrya (formerly Prorodon ) (Hiller, 1993a) and a thicker layer recorded in Cryptocaryon (Colorni & Diamant, 1993; Matthews et al., 1993) and Placus (Grain et al., 1979). Longitudinal microtubules have been observed to lie above the epiplasm in the cortical ridges of Holophrya (formerly Prorodon ) species, a feature demonstrated also in oligohymenophoreans (Hiller, 1993a). The prostome somatic monokinetid can now be characterized definitively as bearing a slightly divergent postciliary ribbon at triplet 9, an anteri- orly directed and typically short kinetodesmal fibril at triplets 5, 6, 7, and a radially oriented transverse ribbon at triplet 4 or 5 (Hiller, 1993a). The kine- todesmal fibril may be bifurcated at its beginning in some species (Fig. 13.3) (Hiller, 1993a; Lynn, 1991). The transverse ribbon microtubules typi- cally extend upward and laterally into the cortical ridge and the postciliary microtubular ribbons may not overlap (Fig. 13.4). However, in Placus , the transverse ribbons extend downward and laterally Fig. 13.3. Schematics of the somatic kinetids of the Class PROSTOMATEA . ( a ) Monokinetid of Coleps . ( b ) Dikinetid of Coleps . ( c ) Dikinetid of Bursellopsis . ( d ) Monokinetid of Bursellopsis (from Lynn, 1981, 1991) Fig. 13.4. Somatic cortex of a typical prostome inter- preted based on the somatic cortex of Bursellopsis . (Modified after Hiller, 1993a.) due to the position of the somatic kinetosomes near the tops of the cortical ridges in this genus (Grain et al., 1979). The kinetids at the anterior of somatic kineties may be dikinetids whose posterior kinetosome bears the same fibrillar associates in similar orientation to those of the somatic monoki- netids . However, the anterior kinetosome bears a single postciliary microtubule and a tangentially oriented transverse ribbon adjacent to triplets 3, 4, 5 (Hiller, 1993a; Lynn, 1991). Hiller (1993a) has suggested a transformation series deriving prostome and colpodean dikinetids from a com- mon ancestor with this somatic dikinetid structure. Microtubules have been observed underlying the kinetosomes of prostomes , originating from dense material at the kinetosome base and extending anteriorly in Holophrya (formerly Prorodon ), the so-called prekinetosomal microtubules (Hiller). Their association with the kinetosome has not been demonstrated in Coleps (Lynn, 1985). Rudberg and Sand (2000) have provided the only study of the electrophysiology of prostomes . They observed a unique biphasic membrane potential that correlates with the alternating periods of linear and circular swimming of Coleps . Mucocysts , typically elongate ovoid, have been observed in the somatic cortex of prostomes (Hiller, 1991, 1993a; Huttenlauch, 1987; Matthews et al., 1993). Colorni and Diamant (1993) observed a bizarre phenomenon in Cryptocaryon : vesicles within the alveoli appear to fuse with the plasmale- mma, presumably releasing material to the outside and leaving the cell\u2019s surface covered with numer- ous \u201cpores\u201d. Mitochondria , often present in the cell cortex, are the typical forms with tubular cristae. As with other ciliates, prostomes have mineral elements in the cytoplasm. Like those in the alveoli of Coleps species, the mineral concretions in the endoplasm have been demonstrated to be calcium carbonate (André & Fauré-Fremiet, 1967; Rieder, Ott, Pfundstein, & Schoch, 1982). There is typically a contractile vacuole in the posterior of the cell, although some larger forms can have about 70 independently functioning vacu- oles