Phylum Chordata
- Created by: rosieevie
- Created on: 06-11-17 13:09
Phylum Chordata
Includes both vertebrates and non-vertebrate chordates
3 sub-phyla:
- Urcochordata (tunicates)
- Cephalochordata (lanclets)
- Vertebrata (inc. lamprey and hagfish)
Chordate Origins
Origin in a tunicate-like (urochordata) ancestral form - Delsuc et al. (2006)
- Cephalochordates and vertebrates considered most related - morphological similarities and increased complexity
- Grouping of tunicates and vertebrates into clade Olgactores
- Structure of cadherin genes
- Limited by relatively few characters considered
- Other studies limited by no. chordate species studied
- Delsuc - extended data to 13 chordates
- Provided evidence that urochordates closest living relatives
- Phylogenetic analyses
Chordate Radiation
Greater variety of habitats than any other phylm, established dominance in some
EX - bony fish
Groups have invaded land then returned to sea e.g. cetaceans
Complicated mechanisms/behaviour:
- Resisting dessication
- Skeletal support (endoskeletons) - no limiting growth
- Some endothermic - optimise muscular activity
- Developed nervous systems - quick adaptation to environmental changes
= Well-established and top of food chains
Chordate Embryology
Deutrostomes (ecinoderms are only invertebrate deutrostomes) - first blastopore opening becomes anus
Principle embryo characteristics:
- Stiff notochord
- Single dorsal hollow nerve cord (component of nerve system)
- Pharyngeal slits (gill slits/throat structures)
- Triploblastic layers (ectoderm, mesoderm and endoderm)
Somites - blocks of muscle which attach to notochord/spinal chord
Notochord
Always found at some embryonic stage of all chordates
First part of endoskeleton to develop
- Give bilateral symmetry
- Axis for muscle attachmment - anchorage
- Some degree of flexibility
May persist throughout life or lost in metamorphosis
NOT A PRECURSOR OF VERTEBRAE (derived from mesoderm cells lateral to notochord)
PERSISTS AS INTERVERTEBRAL DISCS (displaced by vertebrae)
Structure - concentric rings of 3 layers:
- Elastic outer covering
- Fibrous notochordal sheath - oreintation changes = resists forces from all directions = flexibility
- Vaculoated cells in center = hydrostatic core = elasticity
Hollow Dorsal Nerve Cord
Single cord
Dorsal of alimentary canal (digestive tract)
Ventral in invertebrates
Anterior end enlarges to form brain
Neurulation:
- Notochord induces overlying neuroectoderm to invaginate on dorsal side
- Neural tube formed
- Nerve cord produced
Pharyngeal Pouches/Slits
Pharyngeal slits lead from pharyngeal cavity to outside of the body
Formation:
- Inpocketing of ectoderm
- Outpocketing of pharynx endoderm
- Join in aquatic chordates = slit for filter feeding and gills
- 3 grooves persist in amniote chodates = Eustachian tubes, middle ear cavity, tonisls, parathyroid glands
Endostyle
Endostyle - organ assisting lower-chordates (uro, cephalo, lamprey larvae) in filter feeding
- Longitudinal ciliated fold in vental pharyngeal wall
- Secretes mucus - accumulate food particles
- Passes particles to digestive tract
Evolved to form and secrete proteins which bind iodine
- Evolutionary origin of the vertebrate endocrine gland
- Tyrosine tightly bound to iodine - form called MIT in chordates
- Acts as a monomer for condensation polymerisation in active thyroid hormone (T3/T4)
- Associated with region 7 of endostyle
Post-Anal Tail
Continues after the anus
Has notochord and nerve cord running into it
Works with myomeres and notochord = mobility and stability
Propulsion in lower chordates but vestigial in later lineages
Urochordates
Tunicates - sea squirts, salps and larvaceans
Exoskelton tunic
Filter feed with perforated pharyngeal basket
Adults metamorphosis - tadpole-like larvae exhibit key chordate features (notochord, dorsal hollow nerve cord and post-anal tail)
Larvae swim from hatching site and settle - planktotrophic
Larvacea group are free-living - retain chordate features
Cephalochordata
1 extant Genus Branchiostoma (lancelets)
- Well defined head
- Swim with fish-like locomotion using straited myomeres
- Myomeres arranged longitudinally around notochord
- Mostly partially buried
- Filter feed through pharyngeal basket
Demonstrate intermediate features between invertebrates and vertebrates:
- Body wall - epidermic of columnar epithelium and connective tissue (associated w/ muscus layer in vertebrates - protection)
- Myomeres - chevron shaped blocks, reduces the coelom
- Notochord - major role in locomotion as anchorage for muscles
- Caugal fin - propulsive fin, also a storage organ for nutritional reserves for gametes
Branchiostoma Circulatory System
No heart - use 4 vesselled circulation powered by smooth muscle
Peristaltic sequential contraction
Closed system and flow similar to fish - partial system as blood is linked to 2 major organs (intestinal arteries and caecal - head - area)
No blood pigment cells
Functions mainly as nutrient distribution
Diffusion may occur across gills but most from wall of metapleural folds (thin flaps of body wall anterior to atriopore)
Branchiostoma Excretory System
Numerous protonephridia - accumulate nitrogenous waste
- Grouped in large numbers
- Analagous to start of the kidney
- Structural similaries to invertebrate solenocytes - probably convergent evolution
Removed via nephridioduct through pore in atrium
Branchiostoma Nervous System
Dorsal nerve cord - extends most of body
Anterior slight expansion, cerebral vrsicle at base of oral hood
Segmentally arranged nerves arise from cord along body - typical vertebrate pattern of dorsal and ventral roots - serves myomeres to control blood movement
Simple senses - tactile, chemoreceptors and ocelli (eye spots)
Branchiostoma Reproduction
Dioecious
Gonads ventral to pharynx
Gametes exit via atriopore
External fertilisation
Deutrostome development
Origin of Vertebrates
Centrum functionally replaces notochord
Notochord becomes central part of intervertebral discs (nucleus pulposus)
Tissues surrounding it ossifies - patches which become precursors to vertebrae
Vertebral arch - circle of bone around canal through which spinal cord passes
Centrum - solid part of vertebra, to which arches and processes are attached
- Vertebral arches - arisen in stem group giving rise to agnathans and gnathostomes
- Centra appear later - only gnathostomes
Craniata External Characteristics
Cephalisation - drives distinct bilateral symmetry and nerve system advancements
- Responsible for success - skull protects adaptable brain
Paired limbs - originate as swimming stabilisers but now evolved to jointed limbs (terrestrial fine movement)
Craniata Internal Characteristics - Living Endoske
Endoskeleton allows continuous growth - avoids problems and size limitation associated with moulting
Jointed scaffolding for muscles (levers) = greater muscle mass = stronger and cushioning from impact
Skull and ribcage protect vital organs
May also have external keratin/bony plates
Cartilage likely first endoskeletal material - faster growth than bone
Bones store minerals (P, Ca) and mechanical strength for terrestrial life
Craniata Internal Characteristics - Circulation
Highly efficient closed circulatory systems
Multi-chambered hearts
Blood cells with haemoglobin
= Greater efficiency of gaseous exchange, digestion and excretion
Craniata Internal Characteristics - Physiology
Complex and intergrated organ systems
Compelx respiratory systems developed from a perforated pharynx in filter feeding developing to a muscular pharynx for water (gills) or air (lungs)
Complete digestive systems w/ accessory glands for enzymes and hormones for rapid processing of large food volume, storage and detoxification
Short digestion times in predators to move quickly - food heavy and impairs movement
Craniata Internal Characteristics - Embryos
Close association betwen renal and reproductive systems - derived from the same tissues
Extraembryonic membrane enclosing the yolk sac produced in 2 ways:
- Anamniotic eggs (fish/ambphibia)
- Protective membranes of embryo produced by reproductive tract of female
- No extensive outer membrane
- Egg not needed to be protected against dessication
- Amniotic eggs (birds, reptiles and mammals)
- 3 additional extraembryonic/postembryonic membranes
- Amnion - surrounds entire contents of egg
- Chorion - protects egg from dessication
- Allantois - provides water to egg
- 3 additional extraembryonic/postembryonic membranes
Craniata Internal Characteristics - Nervous System
Advanced nervous system w/ well-defined brains
Increased sensory, motor and intergrative controls associated with changes in genetics (Hox genes)
Anterior end of nerve cord = tripartite brain (forebrain, midbrain, hindbrain)
Rise of cerebellum - increase in complex behaviour and social interactions
Paired and complex sensory organs
Hox Genes
Regulates the expression of a hierachy of genes - control the process of long-axis body development fron front to back
Duplication of entire hox complex in vertebrates = dominance of brain and evolution of morphological complexity
4 Hox gene clusters in gnathastomes
Vertebrate Evolutionary Pathways
Garstang's Hypothesis of Chordate Larval Evolution
- Paedomorphosis - reproductive in juvenile form
- Larvae failed to metamorphosis into adult form - developed gonads
- Neoteny - retaining some juvenile features in adult form
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