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Hemichordata Invertebrate Chordates 1 Evolutionary Perspective • Echinoderms, Hemichordates, and Chordates are most likely derived from a common ancestor, since they all share deuterostome characteristics. 2 What characterizes a chordate? • • • • A dorsal tubular/hollow nerve cord A notochord Pharyngeal slits or pouches Postanal tail 3 Which are shared by both Chordates and Hemichordates? • Pharyngeal slits • Most have a dorsal (sometimes hollow) nerve cord. This shows that evolutionary ties between Hemichordates and Chordates are closer than between echinoderms and either of these phyla. 4 5 Hemichordates • Hemichordata means “half chord” • Includes: Acorn worms and Pterobranchs • Live in or on marine sediments 6 5 Characteristics of Hemichordates • Marine, deuterostomes with bodies in 3 segements-proboscis, collar, and trunk • Ciliated pharyngeal slits • Open circulatory system • Complete digestive tract • Dorsal (sometimes hollow) nerve cord 7 Class Enteropneusta • Acorn worms-named so because of the proboscis at the anterior end • 75 species, 10-40 cm usually • Most occupy U-shaped burrows along the sandy shore-line • Cilia and a mucous-covered proboscis assist in feeding • http://www.youtube.com/wat ch?v=4rexMwCRUjg 8 Class Pterobranchia • Name means “wing or feather gills” • Around 20 species-Rhabdopleura • Small-0.1-5 mm, often living in secreted tubes in asexually produced colonies • Proboscis is shield-like and secretes the tube • Found mostly in deep oceans of the Southern hemisphere • Use cilia on arms/tentacles to filter and transport food to mouth 9 10 Phylum Chordata • Bilateral symmetry, deuterostomes • Notochord, pharyngeal slits or pouches, dorsal nerve chord, and postanal tail present at some point during development • Presence of endostyle (produces mucous) or thyroid gland • Complete digestive tract • Ventral, contractile blood vessel (heart) 11 Phylum Chordata • Notochord-a supportive rod extending most of body length • Pharyngeal slits-series of openings along pharyngeal region. Invert chordates use for filter feeding. Some chordates for gas exchange. • Dorsal hollow nerve cord-runs along length of body, associated with development of complex sensory systems • Postanal tail- extends posterior to the anus 12 SubPhylum Urochordata • Marine animals; some species are solitary, others are colonial. • Sessile adults, but motile during the larval stages • Possess all 5 chordate characteristics as larvae • Settle head first on hard substrates and undergo a dramatic metamorphosis (e.g., tail, notochord, muscle segments, and nerve cord disappear) 13 Adult body covered by an outer envelope or tunic; composed of proteins, salts, and cellulose • Tunic encloses a basketlike pharynx, that is perforated by gill slits •http://www.youtube.com/ watch?v=ti0ynaC0R-0 • Tunicates are filter feeders; Oral siphon (mouth)-plankton is trapped in a sheet of mucus and cilia later direct the food-laden mucus to the stomach 14 • Water leaves the animal via an excurrent siphon SubPhylum Cephalochordata- 45 species • “Head chord” •Exclusively marine, tadpole-like animals (up to 5cm) • Although they are capable of swimming, they usually are buried in the sand with only their anterior end being exposed •World’s oceans in shallow waters clean sands •http://shapeoflife.org/video/animation/chordate-animationamphioxus-vertebrate-body-plan 15 SubPhylum Cephalochordata cont. •All chordate characteristics are present throughout their life history • They are filter feeders: inside of the oral hood is lined with cilia wheel organ • These cilia, plus cilia in the pharynx help generate a water current • Water and suspended food particles pass through the oral hood, equipped with projections called cirri that strain larger particles • Feed by secreting a mucous net across the gill slits to filter out food particles that are present in the water. 16