Animal Characteristics

• Animals are eukaryotic, multicellular, heterotrophic organisms with cells specialized to perform specific functions.

  • Animals have diverse body plans.

  • The body plan is the basic structure and functional design of the body.

• Most animals are capable of locomotion at some time during their life cycle, can respond adaptively to external stimuli, and can reproduce sexually.

• In sexual reproduction sperm and egg unite to form a zygote.

  • The zygote undergoes cleavage, a series of cell divisions that produce a hollow ball of cells called a blastula.

  • Most animals develop into a larva, a sexually immature form that may appear and behave differently from the adult.

  • Larvae typically go through metamorphosis, a developmental process that converts the immature animal into a juvenile form that grows into an adult.

Adaptations to Ocean, Freshwater, and Terrestrial Habitats

• Marine environments have relatively stable temperatures, provide buoyancy, and provide readily available food.

  • Fluid and salt balance are more easily maintained in sea water than in fresh water.

  • Currents and other water movements can be a disadvantage.

• Freshwater offers a less constant environment and less food than sea water.

  • Because freshwater is hypotonic to tissue fluid, animals must osmoregulate.

• Terrestrial animals must have adaptations that protect them from drying out and from temperature changes, and that protect their gametes and embryos.

Animal Evolution

• Based on molecular data, biologists hypothesize that most animal clades actually diverged over a long period during the proterozoic eon.

  • During the Cambrian radiation, new animal body plans rapidly evolved among clades that already existed.

• Hox genes control early development in animal groups.

  • These genes had evolved by the beginning of the Cambrian period, and mutations in these genes could have resulted in rapid changes in animal body plans.

Reconstructing Animal Phylogeny

• Biologists hypothesize that cnidarians (which have radial symmetry) and ctenophores (which have biradial symmetry) are more closely related to each other than to animals that exhibit bilateral symmetry.

• Cephalization, the development of a head, evolved along with bilateral symmetry.

• Biologists have also inferred relationships based on level of tissue development and type of body cavity.

  • Embryonic tissues, called germ layers, include the outer layer, ectoderm, which gives rise to the body covering and the nervous system; the inner layer, endoderm, which lines the gut and other digestive organs; and a middle layer, mesoderm, which gives rise to muscle, skeletal structures, and most other body structures.

• In bilateral animals the type of body cavity has been used to classify animals.

  • Acoelomate animals have no body cavity, and coelomate animals have a true coelom, a body cavity completely lined with mesoderm.

  • Some animals have a pseudocoelom (literally, a “false cavity”), a body cavity that is not completely lined with mesoderm.

• Two major evolutionary branches of bilateral animals are protostomes (mollusks, annelids, and arthropods) and deuterostomes (echinoderms and chordates).

• Protostomes undergo spiral cleavage, in which early cell divisions are diagonal to the polar axis.

  • Deuterostomes undergo radial cleavage, in which the early cell divisions are either parallel or at right angles to the polar axis, so the cells lie directly above or below one another

• Protostomes undergo determinate cleavage, in which the fate of each embryonic cell is fixed very early.

  • Deuterostomes undergo indeterminate cleavage, in which early in development each cell has the potential to develop into a complete organism.

• In protostomes the blastopore, the opening from the embryonic gut to the outside, develops into the mouth; in deuterostomes the blastopore typically becomes the anus.

• Molecular systematics has confirmed much of animal phylogeny that was originally based on structural characters, including the axiom that animal body plans usually evolved from simple to complex.

  • However, molecular systematics has also provided evidence for exceptions.

• Based on molecular data, biologists now subdivide the protostomes into two clades: Lophotrochozoa and Ecdysozoa.

  • The Lophotrochozoa include the flatworms, ribbon worms, mollusks, annelids, rotifers, and animals that have a lophophore, a ciliated ring of tentacles surrounding the mouth.

  • The ecdysozoa, animals that molt, include the nematodes and arthropods.

  • The third clade of animals, Deuterostomia, includes the echinoderms, hemichordates, and chordates.

  • These animals have radial, indeterminate cleavage and pharyngeal slits.