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..