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Bio 216 Crazy Chordates Objectives:Discuss the differences between traditional and cladistic classification of chordates, present the major unifying characteristics of the Phylum Chordata, survey the Subphyla Urochordata and Cephalochordata. Phylogenetic tree of chordates Indicates time scale. 1 Protostomes vs Deuterstomes Deuterostomes Blastopore forms anus Radial cleavage direct development Regulative embryo Evolutionary experimentation Symmetry Fig. 3.10 Bilateral symmetry- can be divided along saggital plane into two mirrored portions Organism must have a head (cephalization) 2 Echinodermata Phylogeny Hemichordata Deuterostomes Chordata Embryonic development patterns indicate shared common ancestor with echinoderm-hemichordate group. Phylum Chordata Five distinguishing characteristics of Phylum Chordata. 1. Notochord 2. Dorsal hollow nerve cord (DHNC) 3. Pharyngeal pouches 4. Post-anal tail 5. Endostyle Sometimes only present in larval stage. Notochord Notochord = flexible, rodlike body of cells enclosed by a fibrous sheath. First part of endoskeleton to appear in the embryo. Supports and stiffens body. http://courses.biology.utah.edu/nielsen/3315/html/skeletal/STnotochord.html 3 Serves as an axis for muscle attachment, bends without shortening Undulatory movements Notochord is center of developmental organization No notochord = no development In many organisms, notochord disappears Vertebrates, notochord serves as an axis around which vertebrae develop. Notochord dictates dorsal orientation. Notochord removed Ventral become dorsal Two dorsal sides! Dorsal Hollow Nerve Cord Anterior enlarged to form brain, ectodermal origin. Invertebrates: ventral and solid Chordates: dorsal and hollow 4 Pharyngeal pouches and slits Endoderm Ectoderm Pharyngeal pouches/slits lead from pharynx to outside environment. Ectoderm inpockets Endoderm outpockets Pouches Slits Original function: filter feeding in protochordates Watermouthgill slits Food captured in slits by mucous. Derived function: respiration in aquatic vertebrates. Internal gills Eustachian tubes, middle ear cavity, glands in tetrapod vertebrates Endostyle Glandular groove Mucous Thyroid gland http://www.luc.edu/depts/biology/111/amphioxs.jpg 5 Post-anal tail Adaptation for swimming Seen in all developmental stages of chordates. Urochordata - sea squirts Sessile as adults, surrounded by tough, nonliving 'tunic' of cellulose Adults have two and larvae have all chordate characteristics. Filter-feeders Endostyle secretes mucous Anus empties into excurrent siphon Fig. 2.2 B 6 Circulatory system Open Bidirectional blood flow Transports nutrients Nervous system Adults have a dorsal ganglion No DHNC Cross-siphon reflex Development Free-swimming tadpole-like larvae metamorphose into sessile adult Fig. 15.5 7 Tunicate Larva POST-ANAL TAIL PHARYNGEAL GILL SLITS NOTOCHORD DHNC SIPHONS Subphylum Cephalochordata marine lancelets (Amphioxus) exhibit all 5 chordate characteristics. Habitat = sandy bottoms of coastal waters Filter feeders eat plankton Cilia move food, mucous traps food in pharyngeal slits Hepatic cecum = intracellular digestion Water in mouth, out atriopore 8 Circulatory system: closed Ventral aortaBranchial arteries Dorsal aorta Body tissue No heart, blood pigments or blood cells Nutrient transport Dorsal aorta Branchial arteries Ventral aorta Nervous system Nervous system centered around hollow nerve cord above notochord. No brain. Muscle segments innervated Muscle segment= myomere Amphioxus a direct ancestor of vertebrates? NO HEAD! Cephalochordata is a sister group. Synapomorphies 1. Myomeres 2. Closed circulatory system 3. Kidney-like cells 4. Caudal fin 5. Unique embryonic features 9 Notochord Post anal tail Dorsal Hollow Nerve cord Pharyngeal gill slits, endostyle Urochordata (sea squirts) Chordata Cephalochordata (lancelets) Sister Groups Myomeres Vertebrata Summary Phylum Chordata is named for rodlike notochord that forms a stiffening body axis at some stage in the life cycle of every chordate. Chordates share 5 defining characteristics. Urochordate free-swimming larvae possess all chordate characteristics. Cephalochordates are vertebrate sister group. 10 Reproduction Diecious, gametes pass out the atriopore Fertilization external Larvae planktonic Gas exchange at metapleural folds. Metapleural folds 11 12