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The protozoa with kinetides are arguably the ancestors of all multicellular animals. A kinetide is composed of a flagellum or cilium, its basal granule or kinetosome, and associated structures. The latter include microtubules and microfilaments organized in various ways. The kinetide has many functions. It is used for locomotion. Associated with that function is the capture of food or the location of sexual partners. The kinetide also plays a central role in regulating cellular form. To better understand these multiple functions--and sometimes all functions are performed by the same kinetides--we now describe the organizational diversity of kinetides.

There can be one kinetide per cell or there can be thousands of kinetides. The single kinetide can be compounded into rows or joined into membranelles or specialtufts. In the latter two conditions, new organelles with capabilities beyond those of single kinetides are formed. In brief, single kinetides become compound and then complex as they become new organizational units. Furthermore, the kinetosome of the kinetide can be associated (especially in the flagellates) with the nucleus. It can organize the mitotic spindle of the cell and play a key role in cell division. In the ciliates, kinetosomes are not associated with the nuclei, but they are organizing centers for the complex architecture of the outer layers of the cell and detailed studies have shown us how the ciliate cortex plays a role in regulating development and in maintaining cell structures.

The kinetide is an organelle concerned with not only the immediate survival needs of the protozoan--locomotion and food--but also with its organization and development. Research has shown that its role in development is essential to proper expression of the genes. Thus, the kinetide increases the evolutionary potential of the flagellated and ciliated protozoa.

What should be emphasized in the evolution of kinetidal forms are those trends that lead to multicellular animals. As we have said, all animals are consumers--they ingest their food. That food can be other organisms or parts thereof. What-ever the case, the food is presented as chunks, which are ingested and then digested internally, either in food vacuoles or in digestive cavities. The protozoa use food vacuoles, and ingestion is by phagocytosis. Phagocytosis is usually thought of as occurring in amebae, in which the food particles are engulfed and then located internally in a food vacuole. The same process, essentially, occurs in the ciliates. But here food is ingested at a special cortical organelle--the cell-mouth or cytostome. The ciliates are the only protozoa with a permanent mouth. All the others ingest through temporary mouths, which are induced when phagocytosis occurs.