Topic 26: Introduction to chordates

Notochord

A flexible dorsal rod located between the digestive tract and nerve cord that provides structural support and serves as an axis for body organization

Dorsal hollow nerve cord

A hollow nerve cord located dorsally that develops into the central nervous system (brain and spinal cord)

Pharyngeal slits/clefts

Openings in the pharynx that function in filter feeding, gas exchange, or development into head and neck structures

Muscular post-anal tail

A tail extending beyond the anus containing muscles and skeletal elements used for locomotion, balance, or propulsion

Chordate shared derived traits

Notochord, dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail

Embryonic significance of chordate traits

Some chordate traits are only present during embryonic development in certain taxa

Notochord function

Provides flexible structural support and acts as an organizing center during development

Fate of notochord in vertebrates

Replaced by the vertebral column, with remnants remaining as intervertebral discs

Dorsal nerve cord uniqueness

Chordates have a dorsal hollow nerve cord, unlike other animals which typically have ventral solid nerve cords

Neural tube formation

The dorsal nerve cord develops from ectoderm that folds inward to form the neural tube

Pharyngeal slit function in invertebrates

Used for filter feeding by trapping food particles in mucus

Pharyngeal slit function in vertebrates

Develop into gills for gas exchange in aquatic vertebrates

Pharyngeal arches in tetrapods

Present only in embryos and develop into structures of the head and neck

Post-anal tail function

Provides propulsion in aquatic organisms and balance or movement functions in others

Segmentation in chordates

Body is divided into repeated units, especially evident in muscle blocks and vertebrae

Muscle blocks (myomeres)

Segmentally arranged muscles that enable side-to-side movement for swimming

Lancelets classification

Invertebrate chordates in subphylum Cephalochordata

Lancelet body shape

Blade-like and fish-like in appearance

Lancelet chordate traits

Retain all four chordate traits as adults

Lancelet notochord

Extends the entire length of the body

Lancelet lifestyle

Marine, sedentary, and partially buried in sand

Lancelet feeding mechanism

Suspension feeding using ciliary currents and mucus-covered pharyngeal slits

Atriopore in lancelets

Opening through which filtered water exits the body

Tunicates classification

Invertebrate chordates in subphylum Urochordata

Tunicate evolutionary relationship

More closely related to vertebrates than lancelets

Tunicate lifestyle

Mostly sessile as adults, attached to substrates

Tunicate larvae

Free-swimming, tadpole-like stage that exhibits all four chordate traits

Tunicate adult morphology

Simplified body specialized for filter feeding, lacking most chordate traits

Tunicate metamorphosis

Transition from motile larva to sessile adult with loss of chordate features

Tunicate body covering

A tough outer tunic made of polysaccharides

Incurrent siphon

Opening where water enters the tunicate body

Excurrent siphon

Opening where filtered water exits the tunicate body

Pharyngeal basket

Structure in tunicates used for filtering food particles from water

Tunicate feeding

Suspension feeding using mucus to trap food in the pharyngeal basket

Key difference: lancelets vs tunicates

Lancelets retain chordate traits as adults, while tunicates only show them in the larval stage

Evolutionary significance of tunicates

Demonstrate that chordate traits can be transient and primarily expressed during early development

An organism has pharyngeal slits and a tail only during its larval stage and becomes sessile as an adult. What is it?

A tunicate

An organism retains a notochord, dorsal nerve cord, and segmented muscles throughout its entire life. What is it?

A lancelet

Why are tunicates still classified as chordates even though adults lack chordate traits?

Because their larvae exhibit all four chordate shared derived traits

A marine organism is found buried in sand and feeds using mucus and ciliary action. What chordate group is it?

Lancelet (Cephalochordata)

Water enters through one opening and exits through another while food is filtered using a pharyngeal basket. What organism is this?

Tunicate

A mutation prevents formation of the neural tube during development. Which chordate structure is directly affected?

Dorsal hollow nerve cord

If the notochord fails to develop, what major function is lost in the embryo?

Structural support and proper body axis development

Which chordate feature distinguishes them from most other animal phyla’s nervous systems?

A dorsal, hollow nerve cord

An organism uses pharyngeal slits primarily for feeding rather than respiration. Is it more likely a vertebrate or invertebrate chordate?

Invertebrate chordate

Which structure would be reduced or absent in a chordate that does not rely on swimming for movement?

Muscular post-anal tail

A biologist discovers an organism with a vertebral column. What happened to the notochord?

It was replaced by the vertebral column

Why do human embryos show pharyngeal arches even though adults do not have gills?

They develop into structures of the head and neck

An organism shows side-to-side swimming powered by segmented muscles. What structure enables this?

Myomeres (segmental muscle blocks)

If water entering the mouth exits without reaching the digestive tract, which structure is responsible?

Pharyngeal slits

A chordate embryo develops normally, but the tail regresses before adulthood. Is it still a chordate?

Yes, because chordate traits can be present only during development

Why is the dorsal nerve cord considered a key evolutionary innovation?

It develops into a centralized nervous system (brain and spinal cord)

Which organism provides the best model of the ancestral chordate body plan?

Lancelet

Why are tunicates considered evolutionarily closer to vertebrates than lancelets?

Genetic and developmental evidence shows closer phylogenetic relationship

An organism lacks segmentation in adulthood but shows it during development. Is this consistent with chordates?

Yes, segmentation can be reduced in adults

A chordate lacks a tail as an adult but has one during development. What does this indicate?

The post-anal tail is developmentally present but reduced later

Which structure is directly responsible for organizing surrounding tissues during embryonic development?

Notochord

If an organism filters food using mucus and pharyngeal slits but does not move, which group is it most likely in?

Tunicate

A scientist observes a free-swimming larva that later attaches to a surface and loses mobility. What process is occurring?

Tunicate metamorphosis

Why are lancelets considered more “primitive” than tunicates in body plan?

They retain all chordate traits throughout adulthood

Which chordate trait is most directly linked to efficient locomotion in aquatic environments?

Muscular post-anal tail

An organism has a dorsal structure that becomes the brain and spinal cord. What is the precursor?

Dorsal hollow nerve cord

If pharyngeal slits are repurposed into jaws and ear structures, which group is being described?

Tetrapods (vertebrates)

Which organism would you expect to have an incurrent and excurrent siphon?

Tunicate

Why is the notochord critical before the skeleton forms in vertebrates?

It provides early structural support and developmental signaling

An organism lacks all chordate traits in adulthood but is still classified as a chordate. What must be true?

It had all chordate traits during its larval stage