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Section 26-1: Introduction to the Animal Kingdom
An animal is a multicellular, eukaryotic heterotroph whose cells lack cell walls.
Animals are specialized to carry out the following essential functions: feeding, respiration, circulation, excretion, response, movement, and reproduction.
In general, complex animals tend to have high levels of cell specialization and internal organization, bilateral body symmetry, cephalization, and a body cavity.
Section 26-2: Sponges
Sponges are classified as animals because they are multicellular, heterotrophic, have no cell walls, and contain a few specialized cells.
The movement of water through a sponge provides a simple mechanism for feeding, respiration, circulation, and excretion.
Section 26-3: Cnidarians
Cnidarians are soft-bodied, carnivorous animals that have stinging tentacles arranged in circles around their mouth. They are the simplest animals to have body symmetry and specialized tissues.
Cnidarians typically have a life cycle that includes two different-looking stages, a polyp and a medusa.
Cnidarians include jellyfishes, hydras and their relatives, sea anemones, and corals.