The Ultrastructure of Placus striatus and a Revision of the Family Placidae (Ciliophora)

The first ultrastructural description of the genus Placus is presented. Each somatic kinetosome has a cone‐shaped axosomal plate, nonoverlapping postciliary microtubules, an anteriorly directed kinetodesmal fiber, and a radial ribbon of transverse microtubules, which extend laterally under the ciliary furrow and insert in the cortical ridge. A closed ring of paired kinetosomes encircles the cytostome. A brosse begins adjacent to the oral pairs and extends posteriorly for one‐fourth to one‐half the cell's length. Autapomorphies for Placus include bowling pin‐shaped toxicysts extruded onto a distinct area of the cell surface immediately posterior to the brosse, and a brosse kinety consisting of a single row of paired cilia. Placus and its sister taxon Spathidiopsis both have spiraling kineties composed of single cilia inserted into the side of the ciliary furrow. Spathidiopsis can be distinguished from Placus because it has a brosse consisting of two kineties (i.e. one composed of paired cilia, the other of single cilia), a row of rod‐shaped toxicyst‐bearing palps extending around one side of the oral area and along the length of the brosse, and a mid‐cell cortical inpocketing containing toxicysts and a segment of the brosse. A revised listing of species assigned to the family Placidae is given.