giant tube worms, and the concept is quite similar up until that point.
Folliculina (referred to in the good ol' days as the "bottle-animalcule"), Folliculinopsis sp., a heterotrich like the giant Stentor:
Folliculinopsis. The two long 'wings' or 'ears' sticking out are its peristomal lobes, which can be seen in the preceding SEM. (Ji et al. 2004 J Ocean Univ China)
Folliculinopsis is host to countless bacterial symbionts; in fact so lushly the bacteria thrive on it that one can barely see the ciliate beneath them! Presumably, these bacteria may be involved in chemical defense, protection from the rather toxic surrounding environment or assist in metabolism. Symbiosis with prokaryotes seems to be fairly common for eukaryotes living awkward (extreme) environments, in large part because prokaryotes are simply amazing at biochemistry unlike their metabolically-challenged nucleated counterparts.
SEMs and TEM of symbiotic bacteria on Folliculinopsis sp. The lorica is covered mostly with filamentous bacteria (top left) whereas the surface of the ciliate is entirely covered with coccoid and rod-shaped episymbionts (bottom two SEMs). Moreover, the inside of the ciliate is full of bacteria-containing vacuoles, as seen in the TEM (near the cortex). (Kouris et al. 2007 Mar Ecol)
In another folliculinid, Eufolliculina, the surface of the peristomal lobes has a peculiar feature: short membrane-covered pins at the base of each cilium. Mulisch (1991 Cell Tissue Res) proposes these pins may act as sensory organelles, perhaps to transmit oriented mechanical stimuli. The cilia have a swelling at the level of the pin, filled with peculiar granular particles with potential involvement in calcium regulation (as you may recall from intro-level physiology, Ca2+ is quite popular in signaling systems). Similar cilium-pin complexes have also been found in other folliculinids, suggesting it may be a shared feature.
Cilia with sensory pegs at the base (arrows). (Mulisch 1991 Cell Tissue Res)
The cilium-peg complex reminds me of sensory hairs or sensilla on insects. Mulisch relates it to the hydrozoan cnidocil in the cnidocyst, or the stereocilia (microvili) at the base of the kinocilium in vertebrate sensory hair bundles. Perhaps this is yet another instance of ultimate convergence, as there is ultimately a finite number of ways particular functions can be performed, and evolution's random walks are bound to chance upon some more than once.
The biology of protist sensory mechansims and overall behaviour is still vast, mysterious, murky territory desperately in need of serious investigation. Unicellular organisms have complex behaviours just like multicellular ones, and are no more 'mere automatic responders to stimuli' than we are (due to our cumbersome complexity, much more random noise tends to creep in; perhaps where creativity comes from...); somehow, without a brain or even a nervous system, many unicellular organisms are nevertheless quite capable of performing complex behaviours in response to various stimuli.
This topic was quite popular in the early 20th century, but seems to have been largely abandoned today (in unicellular organisms). Considering the volumes of papers published daily on cell motility in tissue cultures, would it be too much to ask for some investigation of more intelligent cell types, ie. those that also act as entire organisms? Surely a ciliate must be much more fascinating to work with than some confused helpless cells ripped out of context in some suspension? There's enough work to do in this corner of science to keep us busy for many more years to come...!
On that note, the sun is rising. I should respond to the stimulus. By sleeping... (spent a few more hours scratching my head over some potential centrohelids...freaking gaps in the literature are really annoying, especially when you can't access half of it as it lies under piles of dust in some obscure obsolete journals that have been forgotten for the past five decades or so. Fun times.
Ji, D., Lin, X., & Song, W. (2004). Complementary notes on a ‘well-known’ marine heterotrichous ciliate, Folliculinopsis producta (Wright, 1859) Frauré-Fremiet, 1936 (Protozoa, ciliophora) Journal of Ocean University of China, 3 (1), 65-69 DOI: 10.1007/s11802-004-0011-1
Kouris, A., Kim Juniper, S., Frébourg, G., & Gaill, F. (2007). Protozoan?bacterial symbiosis in a deep-sea hydrothermal vent folliculinid ciliate (Folliculinopsis sp.) from the Juan de Fuca Ridge Marine Ecology, 28 (1), 63-71 DOI: 10.1111/j.1439-0485.2006.00118.x
Mulisch, M. (1991). Ultrastructure and membrane topography of special ciliary organelles in the ciliate Eufolliculina uhligi (Protozoa) Cell and Tissue Research, 265 (1), 145-150 DOI: 10.1007/BF00318148