文档介绍：Unifying Themes 1. Chordate evolution is a history of innovations that is built upon major invertebrate traits ? They display many of the basic traits that first evolved in the invertebrates: bilateral symmetry, cephalization, segmentation, coelom, "gut" tube, etc. 2. Chordate evolution is marked by physical and behavioral specializations ? For example the forelimb of mammals has a wide range of structural variation, specialized by natural selection 3. Evolutionary innovations and specializations led to adaptive radiations - the development of a variety of forms from a single ancestral group Characteristics of the Chordates ? Notochord, dorsal hollow nerve cord, pharyngeal gill slits, blocks of muscle, postanal tail Characteristics of the Chordates The notochord ? All chordate embryos have a notochord, a stiff but flexible rod that provides internal support ? Remains throughout the life history of most invertebrate chordates; among, present only in the embryos of vertebrate chordates Characteristics of the Chordates cont. Dorsal Hollow Nerve Cord (=Spinal Cord) ? A fluid-filled tube of nerve tissue that runs the length of the animal, dorsal to the notochord ? Present in chordates throughout embryonic and adult life Characteristics of the Chordates cont. Pharyngeal gill slits ? Pairs of opening through the pharynx ? Invertebrate chordates use them to filter food ? Juvenile fishes use them to them for breathing ? In adult fishes the gill sits develop into true gills ? In reptiles, birds, and mammals the gill slits are vestiges, occurring only in the embryo Characteristics of the Chordates cont. Blocks of Muscle - Myotomes ? Surrounding the notochord and nerve cord are blocks of muscle - myotomes Postanal Tail ? The notochord, nerve cord, and the myotomes extend to the tail ? Found at some time during a chordate's development Invertebrate Chordates SubPhylum Urochordata ? Marine animals; some species are solitary, others are colonial. ? Sessile as adu