The opening scene of this clip shows a Daphnia, commonly called a water flea, on the left and a hydra, a sessile animal in the phylum Cnidaria, on the right. Like most cnidarians, a hydra possesses a ring of tentacles that surrounds its mouth. The mouth leads to a gastrovascular cavity with only one opening. Hydra, like most cnidarians, have highly specialized stinging cells called nematocysts. Some of these stinging cells inject a type of venom that paralyzes the hydra's prey, allowing its tentacles to surround and draw the prey into the mouth. The video shows the Daphnia being immobilized and the hydra ingesting the prey into its gastrovascular cavity. The food is then broken down into small particles that are engulfed, by phagocytosis, by the cells lining the gastrovascular cavity. Credit: Courtesy of Graham R. Kent and Rebecca L. Turner, Smith College.
