Raven Johnson Biology Chapter 38 Answers to Review Questions 1. Animals are multicellular heterotrophs that digest their food internally. This distinguishes them from the other kingdoms of living organisms. Animal cells are distinct from plant cells in that they do not have cellulose-containing cell walls.
Vertebrates possess a backbone-a dorsal vertebral column enclosing a dorsal hollow nerve cord.
Invertebrates do not, yet they are by far more numerous than the vertebrates, containing among them all the arthropod and molluscan phyla, among others. Sponges can be put through a sieve and reduced to single cells, which will then reaggregate to form a new individual. Radially symmetrical animals can be bisected in any direction within a single plane, and includes the jellyfishes and ctenophores. Bilaterally symmetrical animals comprise the rest of the invertebrates and invertebrates and can be bisected only in one plane along a single axis, producing two equal halves. Bilateral symmetry allows for cephalization-a concentration of sensory organs at the anterior end of the animal.