Chordates - Characteristics and Classification

CHAPTER 23: CHORDATES Chordates: - Name derived from notochord - includes most animals familiar to most people - Features shared w/ other animals: - - - bilateral symmetry - coelom - complete digestive system ( tube within a tube) - metamerism - cephalization Deuterostomes - Anus develops from blastopore - radial cleavage - coelom formed by fusion of enterocoelous pouches Classification: - * many traditional groups no longer used b/c they didn’t satisfy the requirement of cladistics that only MONOPHYLETIC groups are taxonomically valid - Ex: is protochordata monophyletic or paraphyletic ? - Ex: Traditionally reptilia (reptiles) did not include Aves (birds). Without including - Paraphyletic birds reptilia is _____________ - MAJOR GROUPS: - - Major group 1 ( VERTEBRATES) - distinct head w/ brain and cranium - paired special sense organs - vertebrae Major Group 2 ( GNATHOSTOMATA) - - Major Group 3 (OSTEICHTHYES) - Major Group 4 (TETRAPODA) - bony endoskeleton paired w/ limbs used for terrestrial locomotion - Major Group 5 (AMNIOTA) - Major Group 6 (REPTILIA ) - - Jaws embryo with amnion (fluid filed sac ) skull characteristics Subphyla: - Cephalochordata: lancelets - Tunicata: tunicates - Vertebrata: vertebrates Ancestry & Evolution: - difficult to reconstruct evolutionary history of the earliest chordates because they were probably soft bodied creatures with little chance of being preserved as fossils - instead learned from analysis of early developmental stages - most likely arose some 540 million years ago - ancestors likely free swimming and soft bodied - Characteristics: - 1. Notochord - flexible rod - extends the length of the body - first part of endoskeleton to appear in embryo - hydrostatic organ with fluid in cell vacuoles or in compartments b/w cells - muscles attach to the notochord, & permits undulating movement of the body - persists throughout life in amphioxus and in jawless vertebrates but replaced by bony vertebrae in other groups - - 2. Dorsal tubular nerve cord - dorsal to the digestive tract - hollow tube - anterior end becomes enlarged to form brain in vertebrates 3. Pharyngeal pouches - openings that lead from the pharyngeal cavity to the outside - In Tetrapod vertebrates, pharyngeal pouches give rise to several diff. Structures including the middle ear cavity & tonsils - evolves as a filter feeding apparatus - In fish, capillary network with thin, gas permeable walls improve gas transfer in the pharynx and led to the evolution of internal gills, permitting the pharynx to act as a respiratory organ - 4. Endostyle - secretes mucus that traps small food particles brought into the pharyngeal cavity - some cells in the endostyle secrete iodinated proteins - these cells are homologous with the iodinated hormone - secreting thyroid gland present in adult lampreys & all other vertebrates - in some protochordates and lamprey larvae, the endostyle and perforated pharynx work together to create an efficient filter feeding apparatus - 5. Postnatal tail - provided motility that larval tunicates and amphioxus need for their free swimming existence - clearly evolved for propulsion in water - efficiency increased in fishes with addition of fins - only evident in humans as a vestige ( the coccyx ) but most other mammals have waggable tails as adults - * always found at some embryonic stage; may change or disappear in later stages of life - * all but pharyngeal pouches are unique to chordates: hemochromatosis have them as well Subphylum Tunicata: - ~3,000 species - live in all seas from near the shoreline to great depths - most sessile as adults - named after tough, nonliving tunic that surrounds the animal and contains cellulose - CLASSES: - Class Ascidiacea - sea squirts - most common, diverse, and best known - all present in larvae - only pharyngeal slits and an endostyle remain in adult forms - Feeding & Digestion: - food particles are brought in with water through the incurrent siphon - they are then trapped on a mucous net secreted by the endostyle - the mucus net is carried into the esophagus - nutrients are absorbed, and indigestible wastes are excreted through the anus - Circulation & Respiration: - ventral heart and two large vessels - connected to a system of smaller vessels and spaces serving the pharyngeal basket, where respiratory exchange occurs and other organs - Unique Feature: heart drives blood first in one direction for a few beats, then pauses, reverses its action, and drives velodrome in opp. Direction - Nervous System: - Nerve ganglion and a plexus of nerves - larval stage has a tubular nerve cord, but is reduced to the other ganglion during metamorphoses - - - Distinctive sensory organs absent in adult sea squirts Reproduction: - hermaphroditic - external fertilization Class Thaliacea - barrel or lemon shaped forms with transparent gelatinous bodies - often bioluminescent Class Appendicularia - tiny creatures shaped like a bent tadpoles Subphylum Cephalochordata: - lancelets - slender, laterally compressed, translucent - 3-7 cm long - inhabit sandy sediments of coastal waters around the world - Previously Amphioxus now Branchiostoma - 32 species - spend most of their lives with only their anterior end exposed - has all 5 chordate characteristics - Movement: segmented muscled (myomeres) attached to notochord and used for swimming - - - - Feeding & Digestion: - water enters the mouth - moves into the pharynx and is trapped on the mucus sheet secreted by the endostyle - moved by cilia into the gut Circulation & Respiration: - Closed circulatory system with no heart - respiration occurs over the surface of the body Nervous system: - hollow nerve cord above notochord - simple sense organs including an ocellus - anterior end of nerve cord not enlarged as in the vertebrate brain Reproduction: - separate sexes - external reproduction - direct development from larvae to adults CHAPTER 23 -24 : INTRO TO VERTEBRATES & FISHES Early vertebrate evolution: - the earliest vertebrates were substantially large than protochordates and considerably more active - modification of skeletal structures and muscles permitted increased speed and mobility - the higher activity levels & increased size of vertebrates also required structures specialized in locomotion, capture, and digestion of food and adaptations designed to support a high metabolic rate Musculoskeletal Modifications : - Most vertebrates have an endoskeleton of cartilage or bone - endoskeleton promotes almost unlimited body size - vertebrates have a cranium protecting the brain and vertebrae - disc like centra replace notochord and neural spines proved more area for muscle attachment - segmented muscles (myomeres) are folded into a W-shape allowing powerful control over the body - endoskeleton was likely initially composed of cartilage and later of bone - cartilage grows faster and is more flexible,making it an ideal material for the first skeletal framework of embryos - endoskeleton of living hagfishes, lampreys, sharks , and their kin and even some bony fishes like sturgeons are composed of cartilage - bone formation may have been adaptive for early vertebrates - structural strength of bone i superior to cartilage - makes bone more ideal for muscle attachments areas of high mechanical stress - bone may also have evolved as means of mineral storage and homeostasis - phosphorus and calcium are used for many physiological process in organisms with high metabolic rates - some of the earliest fishes including Ostracoderms and Placoderms were partly covered in a bony dermal armor - Were modified in later fishes as scales - most vertebrates are protected with keratinized structures derived from the epidermis like reptilian scales, hair , feathers , claws, and horns