Figure 1. Light-micrograph pictures of two suctorians.

A, Tokophrya infusionum: side-view phase-contrast micrograph of a live organism with numerous tentacles (t), a stalk (s), an attachment disc (d) and a contractile or pulsating vacuole (pv). Micrograph by Dr Maria A. Rudzinska, reproduced with permission of The Rockefeller University Archives. Bar, 20 m.

B, Heliophrya erhardi: top-view dark-field micrograph of a live organism after feeding. Four bundles of long slender tentacles can be seen. Each tentacle ends in a knob-like enlargement. Internal cellular structures are obscured by the many food vacuoles that fill the cell body. Bar, 100 m.

C, Heliophrya erhardi: Top-view phase-contrast micrograph of a live organism (after metamorphosis without feeding). This specimen has seven tentacle bundles. The attachment plate protrudes from underneath the cell body and surrounds the cell like a ring (r). Four contractile or pulsating vacuoles (pv) are clearly visible, as is the large irregular macronucleus (ma). Micronuclei (arrowheads) are more difficult to differentiate from small cytoplasmic inclusions. Bar, 50 m.

Figure 2. Some details of suctorian cellular organization (all pictures are from Heliophrya erhardi)

A, Structure of pellicle – consisting of cell membrane and two alveolar membranes (between arrowheads) followed by a fibrillar epiplasm (e). The pellicle is traversed by a pit (p). Electron micrograph; bar, 250 nm.

B, Field of kinetosomes adjacent to the epiplasm (e). Electron micrograph; Bar, 100 nm.

C, Longitudinal section through tentacle tip. The axoneme (a) is sectioned longitudinally (see cross-section in Figure 2E). Haptocysts (arrows) are arranged at the periphery of the tentacle knob. (Haptocysts are marked only on a portion of the knob, and some haptocysts are outside of the section plane.) Electron-dense bodies (b) fill the axoneme and the tentacle knob. Electron micrograph; bar, 500 nm.

D, Longitudinal section through haptocyst showing the complex structure of this organelle. Electron micrograph; bar, 200 nm.

E, Cross-section through a tentacular axoneme within the cell body (see text). This picture also shows ‘dense bodies’ (b) and a ‘vesicle with osmiophilic cap’ (v). Electron micrograph; bar, 250 nm.

F, Light micrograph of reproducing Evaginogenida (e.g: Heliophrya). A large brood cavity (bc) extends through the cell body (see Figure 3, fourth column). The bean shape of the cavity corresponds to the bean shape of the resulting swarmer after evagination. During reproduction, the normally elongated macronucleus becomes round in preparation for nuclear division (ma). Light micrograph, live organism; bar, 50 m.

G, The wall of the Evaginogenida brood cavity. Cilia project from the cell body into the lumen of the brood cavity (bc). Most cilia are pictured as cross-sectional profiles. The arrowhead points to a tentacular axoneme within the cytoplasm. Electron micrograph; bar, 1 m.

H, Organism forming a cytoplasmic bulge in direction (arrow) of a neighbouring cell (not shown). Bulges from two cells contact and fuse, thereby establishing a cytoplasmic conjugation bridge between the organisms. Light micrograph, phase contrast, live organism; bar, 50 m.

I, Two conjugating cells of Heliophrya. The two organisms are connected by a conjugation bridge (between arrowheads). Some micronuclei are marked by arrows in one conjugant. Light micrograph, phase-contrast Feulgen staining. Bar, 50 m.

J, Higher magnification of a portion of Heliophrya's conjugation bridge at the moment of nuclear exchange and nuclear fusion, the central event in ciliate conjugation. The arrows indicate the direction of movement of one nucleus (migratory nucleus) from one organism into the conjugation partner. Top, nuclear pair at the beginning of fusion. Bottom nuclear pair: fusion nearly completed. Light micrograph, phase-contrast Feulgen staining; bar, 10 m.

Figure 3. Diagrams of modes of reproduction in suctorians (tentacles omitted). Differences are shown between ciliated swarmer formation (reproduction) in Exogenida, Endogenida and Evaginogenida. Within the Exogenida, transitional species can link the closely related modes of reproduction of Exogenida and Endogenida.