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M. R. M. COLLEGE, DARBHANGA DEPARTMENT OF ZOOLOGY ONLINE LECTURE SERISE - 2 ON Origin of Chordates B.Sc Part I- Theory (Paper II, Group - A) K. C. MISHRA ASSISTANT PROFESSOR M.R.M.COLLEGE,DARBHANGA Origin of Chordates Fundamental chordate characters • A dorsal hollow or tubular nerve cord. • A longitudinal supporting rod like notochord. • A series of pharyngeal gill slits. Time of origin • Evolved in late cambrian. • Fully established as vertebrates in Ordovician. Theory for Origin of Chordates • Coelenterate Theory • Annelid Theory • Echinoderm Theory • Neotenous Theory Coelenterate Theory • Proposed by Masterman in 1897. • Diploblastic gastrula gave rise to triploblastic animals. • Diploblast evolved into two line of evolution, protostomia and deuterostomia. • Radial symmetry, coelenteron, cnidioblasts, disappeared and advance characters developed to give rise chordates. • This theory infers that chordates might have acquired higher characters independently. This theory is not acceptable. Annelid Theory • Annelid when inverted resembles with the Ammocoete larva of Lamprey. • Ventral blood vessels of annelid correspond to dorsal aorta of chordates, both in position and in general direction of blood flow. • Heart of annelid correspond to aortic arches of the chordates. • Ventral nerve cord of annelid and its anterior ganglia can be compared with the spinal cord and brain of the chordates. • Like chordates the annelids shows bilateral symmetry, metamerism, lateral coelom, complete digestive tract, closed circulatory system. Objection • Annelids lack gill slits, notochord. • Hemoglobin dissolved in plasma in annelids Echinoderm Theory • Echinoderms are radially symmetrical and show no typical chordate characters. Therefore, larval similar characters are considered. • The bipinnaria larva of echinoderm are free swimming, bilaterally symmetrical, transparent and resemble with tornaria of hemichordates. • Presence of 5 coelomic pouches, enterocoelic coelom, deuterostomous mouth, ciliated bands in loops, a dorsal pore, sensory cilia at anterior end, complete digestive system are similar larval characters. • Phosphagene (creatine) for muscles contraction is same in both the phylum. • Presence of apical plate with eyespots in tornaria larva raises doubts about common ancestry of echinoderm and hemichordate. Similarity of Larval Forms Neotenous Larva Theory • Proposed by Garstang and de Beer (1894). • Auricularia larva of echinoderms became sexually mature and later this neotenic larva gave rise to chordate. • If ciliated bands together with underlying neural tissue of auricularia larva of echinodermates concentrate to form ridges leaving a groove between them and if lips of the groove fuses subsequently, it will give rise tube.it will resembles with the neural system of chordates. Larval Lineage for Origin of Chordates N. J. Berrill (1955) in his book, “The Origin of Vertebrates” has suggested the following larval sequence: Echinoderm - Auricularia Hemichordate – Tornaria Protochordate – Ascidian tadpole Permanently free swimming chordate..

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"Lophophorates" Brachiopoda Echinodermata Asterozoa
Deuterostomes Bryozoa Phoronida "lophophorates" Brachiopoda Echinodermata Asterozoa Stelleroidea Asteroidea Ophiuroidea Echinozoa Holothuroidea Echinoidea Crinozoa Crinoidea Chaetognatha (arrow worms) Hemichordata (acorn worms) Chordata Urochordata (sea squirt) Cephalochordata (amphioxoius) Vertebrata PHYLUM CHAETOGNATHA (70 spp) Arrow worms Fossils from the Cambrium Carnivorous - link between small phytoplankton and larger zooplankton (1-15 cm long) Pharyngeal gill pores No notochord Peculiar origin for mesoderm (not strictly enterocoelous) Uncertain relationship with echinoderms PHYLUM HEMICHORDATA (120 spp) Acorn worms Pharyngeal gill pores No notochord (Stomochord cartilaginous and once thought homologous w/notochord) Tornaria larvae very similar to asteroidea Bipinnaria larvae CLASS ENTEROPNEUSTA (acorn worms) Marine, bottom dwellers CLASS PTEROBRANCHIA Colonial, sessile, filter feeding, tube dwellers Small (1-2 mm), "U" shaped gut, no gill slits PHYLUM CHORDATA Body segmented Axial notochord Dorsal hollow nerve chord Paired gill slits Post anal tail SUBPHYLUM UROCHORDATA Marine, sessile Body covered in a cellulose tunic ("Tunicates") Filter feeder (» 200 L/day) - perforated pharnx adapted for filtering & repiration Pharyngeal basket contractable - squirts water when exposed at low tide Hermaphrodites Tadpole larvae w/chordate characteristics (neoteny) CLASS ASCIDIACEA (sea squirt/tunicate - sessile) No excretory system Open circulatory system (can reverse blood flow) Endostyle - (homologous to thyroid of vertebrates) ciliated groove
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Branches on the Tree of Life DVD – CHORDATES Written and photographed by David Denning and Bruce Russell ©2005, BioMEDIA ASSOCIATES (THUMBNAIL IMAGES IN THIS GUIDE ARE FROM THE DVD PROGRAM) .. .. To many students, the phylum Chordata doesn’t seem to make much sense. It contains such apparently disparate animals as tunicates (sea squirts), lancelets, fish and humans. This program explores the evolution, structure and classification of chordates with the main goal to clarify the unity of Phylum Chordata. All chordates possess four characteristics that define the phylum, although in most species, these characteristics can only be seen during a relatively small portion of the life cycle (and this is often an embryonic or larval stage, when the animal is difficult to observe). These defining characteristics are: the notochord (dorsal stiffening rod), a hollow dorsal nerve cord; pharyngeal gills; and a post anal tail that includes the notochord and nerve cord. Subphylum Urochordata The most primitive chordates are the tunicates or sea squirts, and closely related groups such as the larvaceans (Appendicularians). In tunicates, the chordate characteristics can be observed only by examining the entire life cycle. The adult feeds using a ‘pharyngeal basket’, a type of pharyngeal gill formed into a mesh-like basket. Cilia on the gill draw water into the mouth, through the basket mesh and out the excurrent siphon. Tunicates have an unusual heart which pumps by ‘wringing out’. It also reverses direction periodically. Tunicates are usually hermaphroditic, often casting eggs and sperm directly into the sea. After fertilization, the zygote develops into a ‘tadpole larva’. This swimming larva shows the remaining three chordate characters - notochord, dorsal nerve cord and post-anal tail.
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An Invertebrate Is an Animal Without a Backbone. They Have No Internal Skeleton and Normally Have Soft, Flexible Bodies
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Introduction to Phylum Chordata
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