1. Podophrya sp., studied in this work, feeds on various holotrich ciliates. The tips of the tentacles adhere to the prey. Within a few minutes the cilia of the prey in the region where the Podophrya is attached stop beating. The stoppage spreads from here outwards over the surface of the prey. In some cases the surface of the Podophrya becomes wrinkled. The prey is broken down locally in the region of attachment, material from the prey flows up the tentacles into the Podophrya, and the Podophrya gets bigger, any wrinkles that may have formed becoming smoothed out in the process.
2. Wrinkling of the Podophrya is shown by observations and experiments to be due to an expansion or growth of the body surface. This expansion serves to reduce the hydrostatic pressure within the Podophrya, so that feeding may proceed.
3. It is suggested that expansion of the body surface, coupled with a supposed resistance to inward collapse, might provide suction for feeding. Wrinkling of the surface is ascribed to a local collapse which may be supposed to occur when uptake of food material from the prey fails to keep pace with the expansion.