For many links to mollusk-oriented sites, click here . Pelagic tuna crabs , Pleuroncodes planipes. Tiny krill , a principle source of food for blue whales . They do not have true organ structures, having evolved just enough to develop tissues.
Tentacles often surround the single body opening, which serves as both mouth and anus.
Secreted cells named nematocysts are characteristic of cnidarians. These stinging cells are found in tentacles and on the surface tissue of the animal, and are used for defense and the capture of prey. For more information, see a fine page on cnidarians. All text and photographs copyright Phillip Colla . All rights reserved worldwide.