Chordata Anatomy and Physiology

• Chordata Course Outline: Focuses on chordate features, hemichordates, and chordate classes (Urochordata, Cephalochordata, Vertebrates).
• Learning objectives include describing chordate characteristics, identifying features in hemichordates and protochordates, classifying these groups, and describing their morphology and physiological diversity.

Chordate Features

• Key features to identify include multicellularity, eukaryotic cells, absence of cell walls, specialization of cells, heterotrophic nutrition via ingestion, motility, nervous coordination, cephalization, diploidy with haploid gametes, and symmetry.
• Chordates have triploblastic, coelomate, deuterostome, segmented, and bilaterally symmetrical body plans.

Phylum Chordata

• Named for the notochord, a rod-like structure present at some stage of their life cycle.
• This is a diverse phylum ranging from sea squirts to mammals.

Chordate Morphology

• Chordates are distinguished by having, at some stage of their life cycle:
  • A dorsal hollow nerve cord.
  • A notochord.
  • Pharyngeal slits.
  • An endostyle.
  • A muscular post-anal tail.

Notochord

• This is a rod-like, mid-dorsal structure composed of a fibrous sheath enclosing flattened mesodermal cells surrounded by an elastic sheath.
• The notochord is flexible yet stiff, allowing bending without shortening.
• It provides an axis for muscle attachment and enables undulatory movements.
• In invertebrate chordates and primitive vertebrates, it acts as the primary mechanical axis; in vertebrate chordates, it is replaced by the vertebral column.
• The vertebral column consists of cartilaginous or bony vertebrae around the notochord.

Dorsal Hollow Nerve Cord

• In most invertebrate phyla, the nerve cord is solid and ventral to the gut.
• It forms via in-folding of the embryo's ectoderm.
• The anterior end thickens to form the brain.
• It is dorsal to the gut, parallel to the notochord, and runs through the neural arches of vertebrae in vertebrates.

Post-anal Tail

• It is a muscular structure.
• It varies in form among different groups.
• It is located after the digestive tract (anus/cloaca).
• It functions in locomotion, propulsion, and balance.

Pharyngeal Gill Slits

• Gill slits are formed by the invagination of ectoderm and evagination of the endodermal lining of the pharyngeal gland and are supported by gill bars.
• They form functional openings into the cavity or appear as grooves during development.
• These function in feeding and/or respiration.
• Simple chordates use these to strain food particles from water.
• Complex chordates that are aquatic, use these for gaseous exchange.
• Terrestrial chordates have these in embryos only.
• The Eustachian tube in terrestrial adults is a remnant of the gill slits, equalizing air pressure.

Endostyle

• The endostyle is formed from the endoderm of the embryo in the floor of the pharynx.
• In invertebrate chordates and Agnathan larvae, it secretes mucus to trap food.
• In vertebrate chordates, cells change into follicular epithelial cells of the thyroid gland.

Other Chordate Features

• Monomeric body with cephalization.
• Segmented muscles.
• Segmentation does not extend to the coelom.
• Epidermis lacks cilia or a cuticle.
• Complete gut with a mouth to a non-terminal anus.
• Closed or partly open circulatory system with a ventral heart.
• Many are marine, filter-feeding using mucus and cilia.

Chordate Classification

• Hemichordata
• Urochordata
• Cephalochordata
• Vertebrata

Hemichordata

• Enteropneusta
• Pterobranchia
• Planctosphaeroidea

Hemichordata (Acorn Worms)

• Class Enteropneusta ranges in size from 2 cm to 2.5 m.
• These are marine, bottom-dwelling animals.
• They have a tongue-like proboscis, short collar, and long trunk.
• These have pharyngeal slits, plus dorsal and ventral nerve cords, but no notochord or post-anal tail.

Enteropneusta Features

• Locomotion is achieved by pushing the proboscis and collar forward, pulling the body using longitudinal muscles.
• The proboscis probes for food collected on mucus strands.
• The nervous system consists of a sub-epithelial network/plexus of nerve cells and fibers; thickenings form dorsal and ventral nerve cords.
• The circulatory system includes a mid-dorsal vessel carrying blood anteriorly and a ventral vessel carrying blood posteriorly.
• The excretory structure is the glomerulus.
• These have separate sexes, external fertilization, and a Tornaria larva.

Urochordata (Tunicates)

• Marine animals.
• Larvae are free-swimming and cephalized but acraniate.
• Adults are benthic, sessile, generally cylindrical, and enclosed in a tunic, attached to a substrate, and lack cephalization.

Urochordata Classes

• Larvacea: Adults retain larval form.
• Thaliacea: Barrel-shaped with siphons at opposite ends.
• Ascidiacea: Sea Squirts, barrel-shaped with both siphons at the same end.

Ascidiacea Features

• Degenerative chordates: Adults lack chordate features except the pharynx and endostyle.
• Larvae show all chordate features but lose the tail, dorsal nerve cord, notochord, and segmented muscles during metamorphosis.
• They have a cylindrical body with one end attached to the substrate and the other with mouth (inhalant/incurrent siphon).
• They have a large pharynx and atriopore, but no coelom.
• Food particles are trapped by mucus secreted by the endostyle and moved across the pharynx to the dorsal lamina, then collected in a groove and passed to the oesophagus.
• The life cycle undergoes metamorphosis.
• They have an open circulatory system.
• They respire via diffusion across the pharynx.

Cephalochordata (Lancelets)

• Benthic, marine, burrow in sand, and are the "blueprint" of Chordata.
• The heart is a simple pulsating vessel.
• Tapering body with notochord.
• Lack a distinct head (acraniate).
• The nerve cord is dorsal and hollow.
• No brain.
• They respire through skin.
• The lancelet has segmented muscles, a muscular post-anal tail, pharyngeal slits, and the position of the endostyle at the floor of the pharynx.
• Filter feed with a perforated pharynx.

Cephalochordata Body Systems

• Circulation: Closed system without a heart; the ventral aorta pumps blood anteriorly in the ventral vessel and posteriorly in the dorsal vessel;
• Excretion: protonephridia.
• Reproduction: Separate sexes; gonads burst and shed into the atrium; external fertilization.
• Locomotion: V-shaped segmented muscle blocks (myotomes); can swim forwards and backwards.

Vertebrata/Craniata Features

• Mostly marine organisms include fishes, amphibians, reptiles, birds, and mammals.
• In addition to diagnostic chordate features, vertebrates have a complex nervous system (spinal cord and brain), cranium, complex neurosensory organs, and a spinal column of vertebrae.
• Locomotion occurs via paired limbs or fins.
• Vertebrata have a well-developed coelomic cavity, highly efficient respiratory systems, an endocrine system, and efficient closed circulatory systems with ventral muscular hearts.
• Vertebrates integument includes an outer epidermis and inner dermis.
• These have a complete digestive system with a muscularized gut wall, an efficient excretory system, and reproduce sexually (mostly dioecious).

Vertebrata Classification

• Agnatha (jawless): Cephalaspidomorphi (lampreys), Myxini (hagfishes).
• Gnathostomata (jawed): Chondrichthyes, Actinopterygii, Sarcopterygii, Amphibia, Reptilia, Aves, Mammalia.

Pisces

• Objectives include describing and classifying features of fishes, outlining their evolution, and distinguishing different classes.
• Agnatha consists of classes Petromyzontida and Myxini. Gnathostomata includes Chondrichthyes, Sarcopterygii, and Actinopterygii.
• Jawless fishes appeared in the Ordovician period, jawed fishes in the Silurian, and the Devonian period was the "Age of Fishes".

Agnatha (Jawless Fishes)

• Classes Petromyzontida (lampreys) and Myxini (hagfishes) lack jaws.
• Features of Petromyzontida include a persistent notochord, pharyngeal slits, post-anal tail, dorsal spinal cord, and endostyle in larva.
• Lampreys attach to hosts using a rasping ring of teeth.
• Features of Myxini: Mucous all over body, no vertebrae, craniate, heart + accessory hearts, and they are blind.

Gnathostomata (Jawed Mouth)

• Made up of classes Chondrichthyes, Actinopterygii, and Sarcopterygii.

Chondrichthyes (Cartilaginous Fishes)

• Class Chondrichthyes features cartilaginous skeletons and are mostly marine.
• Divided into subclasses Holocephali (chimaeras) and Elasmobranchii (sharks, rays).
• Sharks, rays have streamlined bodies, leathery skin with dermal placoid scales, no operculum, heterocercal tail fins, ventral mouths with shedding teeth, no swim bladder, two-chambered hearts, and well-developed sensory systems.

Subclass Elasmobranchii (Sharks, Rays):

• Internal structures include a rectal gland maintain osmotic balance and a spiral valve increases intestinal surface area.
• Sharks have Lateral Line Receptors (neuromasts) – detect disturbances in water Ampullae of Lorenzini – respond to weak electric fields, and possibly temperature, water pressure, and salinity.

Osteichthyes (Bony Fishes/Teleosts)

• Classes Sarcopterygii and Actinopterygii, and a bony skeletons.
• They have an operculum covering a single gill opening, scales, and a swim bladder.
• Respiration occurs via gills, lung-like sacs, or swim bladders.

Evolution

• Two great bony fish or teleost classes survive:
  • Class Sarcopterygii = fleshy finned fishes = Coelacanths and Lung fishes (including some Devonian forms that led to Amphibia)
  • Class Actinopterygii = ray finned fishes (including modern bony fishes).

Subclasses of Class Actinopterygii (Ray-Finned Fishes)

• Subclass Chondrostei
• Subclass Neopterygii (includes Teleostei)
• The subclass Chondrostei (e.g., sturgeon & paddlefish) features cartilage and bone.
• Neopterygii (only bone in skeleton);
  • Holostei like bowfin & gar-pike
  • Teleostei, true modern bony fishes, in 9 super-orders

Teleostei

• Streamlined, tapered body with homocercal tail fin.
• Paired fins and unpaired fins.
• Gill cover (operculum).
• Very thin epidermis and a thicker dermis.

Actinopterygii Anatomy

• Fin rays of cartilage or bone supported by pterygiophores.
• S-shaped myomeres.
• The bony vertebral column has many vertebrae.
• Large skull with many bones.

Actinopterygii- Respiration

• Gills supported by bony gill arches, covered by a common operculum, and bearing many gill filaments.
• Ventilation is produced by buccal and opercular pumps, and gas exchange occurs from a water current passing in a single direction: in at mouth, and out over gill lamellae.
• Rete mirabile: Capillaries for countercurrent multiplication to increase O2 concentration
• The swim bladder is present for buoyancy and sometimes respiration.

Teleostei Circulation

• Circulation: Single circuit consists of 1. Atrium (with a smaller chamber,sinus venous) and 2. Ventricle (with a smaller chamber,bulbus arteriosus)
• Alimentary Canal: jaws present. - Mouth terminal with many teeth.

Teleostei Excretion & Osmoregulation

• Excretion & Osmoregulation : Kidneys elongated mass above swim bladder.- Ducts of 2 kidneys join posteriorly and swell to form urinary bladder
• Teleostei Reproduction: Dioecious, gonads paired

Teleostei Sensory Organs

• Have mastery of liquid medium related to ability to receive stimuli by mechanical and electrical means.
• The lateral line consists of neuromasts with sensory cells with hairs embedded in a gelatinous cupula.
• Utriculus, lagena, and sacculus containing otoliths, lymph fluid & hair cells. No external pinna
• Semi-circular canals: Balance
• Avoidance of Predators
  • Locomotion using fins and body wall muscles
  • Anti-predator behaviors in fish

Actinopterygii Evolution

• Body plan allowed for development of range of specialized fishes.
• They were capable of rapid and complex movements, had effective respiratory systems, and could use a wide variety of foods.
• Scales reduced in weight and went from complexity from heavy, rough ganoid to lighter more flexible, bony-ridged scales .
• Original heterocercal tail turned into symmetrical homocercal tail.
• Jaw Structure: From a rigid toothed structure adapted for biting and grabbing came a much more flexible structure.
• Swim bladders went small size and the respiratory function diminished.

Sarcopterygii (Lobe-Finned Fishes)

• Osteichthyes made up of Subclass Dipnoi (lung fishes) and Subclass Crossopterygii (coelacanths).
• Pectoral and pelvic fins resemble tetrapod limbs.
• Respiration occurs via gills and "lungs," with Dipnoi needing to surface and gulp air.

Dipnoi (Lungfishes)

• Extant types, freshwater.
  • African and South American varieties use long, stringy, mobile pelvic and pectoral fins to touch and sense surroundings and have paired lungs;
  • Australian variety has more robust fins and a single lung (one atrophies).

Dipnoi (Lungfishes)

• The reduction includes bone, skull bone numbers, and scale hardness.
• The loss of separation between dorsal, caudal and anal fins (fused).

Crossopterygii (Coelacanths)

• Gill respiration.
• Fleshy extensions form part of ventral fins and may have skeletal elements inside.
• The mtDNA closer to mtDNA of land animals (like frogs) than that of Crossopterygii (coelacanths)
• Lobed fins and tale
• Double jaw suspension

Amphibia

• Objectives include explaining tetrapod evolution, describing amphibian characteristics and classification, comparing morphological features, and outlining anuran development.
• Gas composition, density, and temperature regulation
• Diversity of niches (physiochemical; biotic, substrate)
• Link between semi-terrestrial amphibians and lobefined fishes (Sarcopterygii) evolved from a common bone-scaled, lobe-finned (Osteolepid) fish ancestor.
• Fish-like amphibian ancestor would possibly have lungs and possibly have stoutly constructed fins to crawl..

Evolution of Amphibians

• Eusthenopteron – paddle on mud pools
• Acanthostega – well formed tetrapod limbs but too weak to walk on land Fin rays evolve to become phalanges (digits)
• Feature of terrestrial life: pentadactyl limb of tetrapods. – pentadactyl = five fingered; tetrapoda = four-footed.
• Locomotory actions - like walking, crawling, hopping & jumping - changes in arrangement of muscles & skeleton.
• Pectoral and pelvic girdles provide an articulatory surface for limb for locomotion.

Reptilia

• Objectives include describing terrestrial adaptations, classifying reptiles, describing features, distinguishing Anapsida and Diapsida, describing features of orders, and identifying adaptive radiation.
• Reptiles are fully terrestrial vertebrates facilitated by well-developed lungs, scaly water-resistant skin, and the amniotic cleidoic egg.
• They have bodies of varied shape with heavily keratinized skin covered with horny scales and a few glands.
• Four paired limbs (tetrapod) with 5 toes (pentadactyl).
• Skull with one occipital condyle.
• They are Ectothermic

Characteristics of Reptiles

• Three-chambered heart, but better separation of venous and arterial blood than in amphibians due to septum
• Homodont teeth and jaws for crushing force plus a cloaca.
• Sexes separate with internal fertilization.
• No larval form.
• Amniotic eggs cushion the embryo.
• Amnion enclosing amniotic fluid.
• Four evolutionary lines based on skull types: Anapsida, Synapsida, Parapsida, Diapsida.
• Turtles are Anapsid or Diapsid

Skull Openings (Fossae)

• Anapsida: No openings.
• Synapsida: One lower opening.
• Parapsida: One upper opening.
• Diapsida: Two openings.
• Lepidosauria: Squamata (Snakes, Iguanas), Sphenodonta
• Archosauria features:Antorbital and mandibular fenestra plus: a. no teeth on secondary palate b. bony secondary palate (internal nares far back)

Subclass Anapsida, Order Testudines/Chelonia (Turtles)

• Herbivorous, lack teeth, have nostrils & lungs, low metabolic rate, and high longevity.
• The vertebrate and ribs are fused with carapace and forms a protective box
• Shell.

Subclass Diapsida, Order Sphenodontia/ Rhynchocephalia (Tuataras)

• One genus with primitive diapsid skulls.
• An immobile Quadrate bone
• Median parietal (third) light sensitive “eye”
• No intromittent organ.

Subclass Diapsida Order Squamata (Lizards, Snakes and Worm Lizards)

• Highly kinetic skull with movable quadrate bone.

• 9,400 species in the 2 groups.

• Paired copulatory organs in males.
• Paired organs of Jacobson

Squamata (Lizards)

• changing colour to match environment
• caudal autotomy

Squamata (Snakes)

• Complete absence of limbs.
• Lateral body undulation locomotion.
• The eye structure displays a spectacle (transparent scale of skin).
• The skull is highly modified to permit gape.
• The respiratory system is modified to keep airway clear.

Archosauria

• "Age of reptiles" that existed 250 – 65 m.Y.B.P. incl extinct dinosaurs and pterosaurs today.
• Antorbital and Manibular fenestra
• Hind limb longer than forelimb
• Saurischia dinosaurs have Lizard pelvis while Ornithischia have bird pelvis
  • In deep sockets.
• Hind limb longer than forelimb

Crocodilia (Crocodiles & Alligators)

• Archosauria that have early features with powerful jaws.
• A secondary palate enables breathing with food in the mouth.
• These are amphibious reptiles with social organization.

Aves

• Objectives include classifying birds, describing characteristics, describing evolution of birds, and describing features for aerial living.
• Said to evolve from forms of reptiles with hollow bones and three toes.
• A feathered Early Birds
• Warren’s Truss for weight

Avian Classification

• Archaeornithes
• Superorder Neognathes and Paleognathaes.
• Body fusiform or spindle-shaped and Skeleton fully ossified with air cavities.
• Toothless jaws and A well developed keel with a tough lightweight beak.
• Scales shed periodically where Reptilian-like scales (visible on feet)

Feathers

• Feathers of modern birds are made of keratin.
• Insulation.
• Flight.
• Structure of Modern Feathers include 1. Large primary flight feathers or pennae(on wings) 2. Semi-plume and Down Feather
• Flight Feather (Pennae) structure Large pennae have a central shaft or rachis, and rows of barbs.
• Wings use downstroke: mainly by huge pectoralis major muscle, upstroke: supracoracoideus pulls wing up

Adaptations for Flight

• 4-chambered heart.
• Rapid heart beats. Bones are light, thin and sheet-like
• Air sacs extend into the bones.
• Very low relative density.
• Hollow bones.
• Advantages of flight relate to freedom of movement in the air

Adaptations that help Birds for Flight

• Well-developed circulatory system
• Metabolism and Respiration helps flight
• Bird Respiration Air inhaled from outside to posterior sacs.
• A large Cerebellum
• Excretory system of metanephric kidney – no bladder – semisolid urine.
• Lay Amniotic eggs with Parental care

Mammalia

• Unique features of Mammals
• General features of mammals
• Classifications and Evolution of Mammals
• Mammals have hair, mammary glands, a diaphragm, and alveoli.
• 3 ear ossicles andHeterodont teeth

Dental Morphology

• Differentaited set of Heterodont teeth that include: Incisors, Canines, Premolars, Molars
• Most advanced nervous system, Biconcave erythrocytes,
• Nervous System: A large brain with most advanced Neurosensory organs

Mammalia, Vertebrata, Mammalia Classification

• Class Mammalia
• Subclass Prototheria
• Subclass Theria
  • Infraclass Metatheria
  • Infraclass Eutheria
• Homotherm and Endothermic
• Wide range of climates

Prototheria

• Oviparous and do not have primitive teeth
• Have Common opening for digestive, excretory and reproductive systems

Theria

• viviparous;amniotic egg not cleidoic (no shell)
• Metatherians (marsupials)- rudimental placenta.
• Eutherian mammals have full placenta

Metatheria

• Have new nipples to deliver milk to Marsupials and are born early Subclass Theria Mammalia, Distinguishing features
• Placenta connects the embryo to mother though which also come Chromosomes

Radiation

• All primates share grasp reflex, nails and binocular vision. Ancestry of primates comes from Arboeral tree shrews
• Flexible balance and highly senesed
• Siminans have good hearing with strong muscles.