Download Ch 23: Phylum Chordata

Z203/ Unit 1 Ch 23: Phylum Chordata I. Characteristics ­ Approximately 45,000 species in 8 major classes A. Structural Plan ­ Shared features w/ Invertebrate Phyla: bilateral symmetry, anterioposterior axis, eucoelom, metamerism, cephalization, complete digestion ­ Echinoderm/Hemichordate assemblage may be sister Groups that share: Deuterostomes = mouth 2 o derived coelom formed from pouches in mesoderm ­ name CHORDATA comes from Notochord (rod­like, semi rigid tissue enclosed in connective tissue) ­ Notochord extends length of body & lies between gut tube & nervous system ­ serves to stiffen body or act as skeletal axis ­ Chordates have more structural unity than other phyla, share 5 hallmark characteristics plus genetics
1 II. Five Chordate Hallmarks A. Notochord ­ always found at some embryonic stage of all chordates ­ 1 st part of endoskeleton to develop ­ serves as axis for muscle attachment ­ may persist throughout life (protochordates & jawless vertebrates) or be lost completely w/ metamorphosis ­ series of cartilaginous bony vertebrae form in vertebrates derived from mesoderm cells lateral to notochord ­ most vertebrates, notochord displaced by vertebrae (remains persist as intervertebral discs) B. Hollow Dorsal Nerve Cord (Tubular) ­ single cord dorsal to alimentary canal (ventral in invertebrates) ­ anterior end enlarges to form brain ­ cord produced by infolding of ectoderm on dorsal side C. Pharyngeal Pouches & Slits ­ slits lead from pharyngeal cavity to outside ­ form by inpocketing of ectoderm & outpocketing of pharynx endoderm ­ 2 pockets join in aquatic chordates & form complete slit for filter feeding & respiration
2 ­ grooves persist (pockets don’t join) in amniote chordates (give rise to structures: eustachian tubes, middle ear cavity, tonsils, & parathyroid glands) D. Endostyle ­ mucous secreting structure found in the Pharynx floor (traps small food particles) ­ found in Protochordates & Lamprey larvae ­ cells associated secrete iodine­based proteins (Thyroid gland in Vertebrates) E. Postanal Tail ­ works with muscles (Myomeres) & notochord to provide motility & stability ­ Aids in propulsion in nonvertebrates & fish but vestigial in later lineages III. Ancestry & Evolution A. History ­ earliest protochordates soft bodied (not fossilized) ­ deuterostomes are natural grouping w/ common origin in Precambrian Seas (Echinoderms, Hemichordates, Chordates) ­ anatomical, developmental, & molecular evidence indicates chordates arose 570mya from lineage related to Echinoderms & Hemichordates ­ significant developments occurred within chordates: skull, jaw, amnion, fins & limb arrangement
3 ­ group divided into 3 Subphylum 1. Urochordata 2. Cephalochordata 3. Chordata INVERTEBRATE CHORDATES: IV. Subphylum Urochordata "Tunicates" “Tail Chordates” A. General Characteristics ­ 2,000 species; occur in all seas & at all depths ­ free living larva, most adults metamorphose into sessile ­ only larvae bears all chordate hallmarks; most adults only retain a few EXTERNAL: ­ tunic (tough nonliving test produced by mantle) surrounds body that contains Cellulose ­ adults form has sac­like body with stolons at the base and siphons at the top (may have more than 2 & colonial forms merge their siphons: Incurrent siphon has sensory tentacles Excurrent siphon has pigment spots INTERNAL: ­ flow pattern of water functions in respiration & digestion & excretion (incurrent siphon→pharynx→gill slits→aterial cavity →excurrent siphon) ­ Respiration occurs across pharyngeal gill slits ­ Digestion – extracellular complete using mucous nets secreted by endostyle to capture food particles (incurrent siphon→pharynx→gill slits→esophagus →stomach→intestine→anus→atrial cavity→ excurrent siphon)
4 ­ Open circulatory system with ventral heart + 2 large blood vessels (pumps 2 directions) * unique blood characteristics (RBC without hemoglobin) * blood flow through gill slits aids in respiration & excretion ­ Nervous system – reduction of dorsal nerve cord & ganglia in adult (sessile way of life) ­ notochord & tail disappear in metamorphosis ­ Reproduction – dioecious with larva free swimming (larva retain chordate features) ­ 3 Groups: Ascidians "sea squirts" Thalacians "salps" Larvacea "larval tunicates" (paedomorphic) V. Subphylum Cephalochordata "Lancelets" “Head Chordates” A. General Characteristics ­ 29 species worldwide, 5 species in North American ­ live in sandy bottom of coastal waters EXTERNAL: ­ slender, laterally flattened, translucent, 5­7cm ­ reduced fins each with a fin­ray aid in movement & digging (dorsal, caudal, ventral) ­ fusiform body shape (tapered anterior & posterior) *Amphioxus = “both ends” “sharp” ­ anterior end has rostrum, oral hood, oral tentacles @ mouth INTERNAL: ­ retain the 5 distinct chordate characters in simple form
5 ­ Respiration – flow pattern of water across gill slits ­ Digestion – Complete system with cilia (wheel organ) driving currents for filter feeding (like tunicates) *Endostyle located at floor of Pharynx *Cecum resembles vertebrate pancreas (secretes enz) (tentacles→wheel organ→mouth→pharynx→gill slits (endostyle)→stomach (hepatic cecum)→intestine→anus) ­ Trunk muscles suggest vertebrate pattern (myomeres) ­ Circulatory ­ same basic Closed circulatory plan as vertebrates *no heart – modified arteries for pumping *capillaries in gill slits increase gas exchange for respiration * blood only carry nutrients ­ no RBC or Hg for O2 ­ Excretory ­ several U­shaped Nephridia (exit atriopore) ­ Nervous system consists of ganglia + dorsal nerve cord & spinal nerves * simple senses include tactile & chemoreceptors * ocelli (eye spots) for light detection ­ Reproduction – Dioecious with gonads ventral to pharynx * gametes exit atriopore VI. Subphylum Vertebrata “Craniata” A. General Characteristics EXTERNAL: 1. Cephalization ­ predominance of head and its development drive distinct bilateral symmetry & nervous system advancements
6 2. Paired Limbs ­ pectoral & pelvic appendages originate as swimming stabilizers ­ jointed limbs suited for terrestrial life & permit fine movement INTERNAL: 1. Living Endoskeleton ­ internal location departs from invertebrate pattern ­ grows w/ body, not confining nor molted ­ greater economy of building materials ­ provides jointed scaffolding for muscles; muscles cushion it from impact ­ skull & ribcage enclose & protect vital organs ­ bony plates , tough integument protects (exoskeleton) ­ cartilage likely 1 st endoskeletal material *superior to bone for fast growth) ­ cartilage remains 1 o endoskeletal material for Agnathans & sharks ­ bones offer advantage of storing minerals (P,Ca) & mechanical strength for terrestrial life 2. Physiology – system become more complex ­ Respiration – perforated pharynx originated for filter feeding * evolved to a muscular pharynx for pumping water * shifted function from feeding to respiratory (gills or lungs) ­ Digestion – Complete system with more specialization * accessory digestive glands (liver & pancreas) needed for larger food processing
7 ­ Circulatory – Closed system w/ multiple chambered hearts and blood components * RBC with hemoglobin aid in O2 dispersal * Aortic arches dedicated to gill slits for gas exchange ­ Excretory ­ paired kidneys remove metabolic waste products 3. Advanced Nervous System ­ switch to active predation favored sensory, motor & integrative controls ­ anterior end of nerve cord became tripartite brain (forebrain, midbrain, hindbrain) ­ selected for sensory organs: * paired eyes w/ lenses & inverted retinas, * paired ears, * receptors (pressure, electro & chemo) ­ head & paired sense organs possible due to derivatives of ectoderm germ layers: *neural crest (ectodermal cells) develops into structures (cranium, teeth, ganglia, endocrine glands, & some nerves) *epidermal placoides (plate like ectodermal thickenings) give rise to olfactory r/c, eye lens, inner ear epithelium, & receptors (taste, photo & lateral line mechano)
8 ­ significant changes in genetics (duplication of Hox genes) drove changes in nervous system and entire body *1 copy of Hox genes in Amphioxus * 4 copies in Gnathostomes VII. Evolutionary History A. Ancestral Invertebrate Chordates ­ ribbon­shaped fish from Mid­Cambrian are oldest invertebrate Chordate fossils seen (similar to lancelets) ­ Garstang's Hypothesis of Chordate Larval Evolution (1928) Paedomorphosis: larvae failed to metamorphosis into adult form, but developed gonads & reproduced in larval stage ­ Lancelets diverged early from shared common ancestor w/ tunicates & many consider them sister group & closest living relative to vertebrates ­ Lamprey larva (ammocoetes) were once believed to be adult lancelets due to similarities
9 B. Earliest Vertebrates 1. Superclass Agnatha: Jawless Ostracoderms articulated fossils found from late Cambrian (530mya) heavily armored (bones in dermis) extinct fish (lasted 150mya) lacked lateral fins for stabilization rudimentary vertebrae fixed circular mouth 2. Superclass Agnatha: Modern Jawless Fish naked hagfish and lampreys classification based on missing trait ­ paraphyletic 3. Gnathostomes: Early Jawed Vertebrates all have jaws = modifications of 1 st two cartilaginous gill arches derived from neural crest (not mesoderm like most bones) jaw musculature homologous to gill musculature most likely monophyletic group Placoderms (early Devonian) among 1 st jawed vertebrates armored fish covered w/ diamond shaped scales or large plates great diversity arose but went extinct by Devonian period grew to 10m length Acanthodians gave rise to modern bony fish 4. Modern Fish & Tetrapods ancestry uncertain unknown from which chordate group vertebrate lineage originated
10 C. Classification of Phylum Chordata Protochordata (Acrania) Subphylum Urochordata (tunicates) Subphylum Cephalochordata (lancelets) Craniata Subphylum Vertebrata Superclass Agnatha Class Myxini (hagfish) Class Pteromyzontida (lamprey) Superclass Gnathostomata Class Chondrichthyes Class Actinopterygii Class Sarcopterygii Class Amphibia Class Reptilia Class Aves Class Mammalia
(sharks, rays, skates) (ray­finned fish) (lobe­finned fish) 11