Figure 20.5: Diversity among the Ciliates (A) Tetrahymena thermophila is used as a model organism for research on gene expression as well as the structure and function of microtubule arrays. (B) The barrel-shaped Didinium nasutum feeds on other ciliates, including Paramecium. Their cilia occur in two separate bands. (C) Some of the cilia in Euplotes fuse into flat sheets that direct food particles into an oral groove.