BIO 123 Lecture Notes - Lecture 12: Polyphyly, Ontogeny, External Fertilization
Document Summary
Hollow ball of cells in embryonic development. Body symmetry is a key morphological aspect of an animal"s body plan. Radial symmetry: they have atleast two planes of symmetry. Arised earlier in the evolution of animals than bilateral. Sea stars, sea urchins, feather stars, and brittle stars. Bilateral symmetry: two sided symmetry, one plane of symmetry and tend to have a long, narrow body. Evolution of a head, or anterior region. Structures for feeding, sensing the environment, and processing information are concentrated. Large mass of neurons in the head. Processes information to and from the body. Once thought to be mollusks, then deuterostomes. Mouth at one end, anus at the other end. Outer tube forms the nervous system and skin. Mesoderm in between forms muscles and organs. Can move effectively even if they do not have fins or limbs. Acoelomates: bilaterians that subsequently lost their coelom, no cavity.
