davoid

The role of consumer or ingestor is also that of predator, which usually means that the predator must be larger than the prey; otherwise, ingestion is impossible. This puts a selective advantage on increase in size, at least for certain predators. That has many functional as well as evolutionary complications. Increase in size means new solutions to locomotion. The kinetide solves that by becoming compound and then complex in response to selection pressures. Increase in size means an adjustment of nuclear content to cells needs. Various solutions are found. Large flagellates (and ameboid forms) become multinucleate. Large ciliates develop a macronucleus, a highly polyploid structure. Large size also demands coordination of various parts of the cell body. The microtubules and microfilaments of the kinetide as well as associated membranes seem to act as coordinators. In addition to coordination of body parts, a predator must coordinate sensory input with body function. Predators must be able to locate prey, capture it, and ingest it. In the ciliates, especially, the kinetides aid in all this. Ciliates do respond chemotactically and thigmotoctically (by touch) to prey in their vicinity. They swim toward their prey. Prey is swept into the cytostome or captured by other special cortical organelles, and then ingested.

This predatory behavior puts a premium on body specialization, which includes sensory, locomotory, and ingestatory apparatuses, all within a single cell. It demands a complexity of the cell body not found in any other living thing. The protozoa are the most complex of all cells, and the ciliates are supreme among the protozoa. We see in them the consequences of the selective pressures arising from a predatory mode of life. In the multicellular animals this means bilaterally symmetrical forms with specialized anterior ends. Theanterior ends typically carry special sensing devices--eyes, ears, noses, taste buds--and ingestatory structures--jaws and mouths. Bilateral symmetry allows these predators to develop the specific orientation--right and left, up and down--necessary for predation. All these evolutionary innovations are anticipated by the ciliates. They will be discussed in further detail when we come to metazoan origins. Now let us make some concluding comments on kinetidal protozoa.

The flagellates and ciliates get larger, develop more complex kinetidal and nuclear apparatuses, and become more complex; this complexity includes permanent diploidy and polyploidy in the ciliates. The key step, in going from the zooflagellates to the ciliates, is the emergence of a permanent mouth, a hallmark of predation. This neosemic character may well explain why most of today's zooflagellates are symbiotic and why the ciliates are mostly free swimming. The now more efficient predators, the ciliates, eliminated their ancestors, the zooflagellates, except in cases in which the latter had invaded specialized niches. These niches appear today as host organisms. But note, when the host organisms are multicellular animals, those hosts evolved after the appearance of the protozoa. Presumably the zooflagellates survived in special, free-living niches and then when opportunities for symbiosis arose, they took them. Extant symbiotic zooflagellates can therefore be viewed as relict populations of once widespread free-living species, now replaced by the ciliates.