Science / Biology / Anatomy ;Deuterostomes latest updated exam questions with verified solutions 2025

Deuterostomes

- Animals in which the blastopore becomes the anus during early embryonic development

- Radial cleavage - cleavage planes are parallel or perpendicular to the vertical axis of the embryo

- Embryos are regulative so the cell fate is not yet determined

- Indeterminate cleavage

- Echinodermata

- Hemichordata

- Chordata

Deuterostome blastopore

- Dividing cells first form a solid ball of cells, the morula, from a zygote

- Eventually a cavity develops and the ball becomes hollow, blastula

- Blastula folds in on itself during gastrulation

- In protostomes, the blastopore forms a mouth before the anus

- In deuterostomes, the blastopore develops the anus before a mouth

Deuterostome coelom formation

- Folds of the archenteron form the coelom

- Evagination of archenteron roof in many deuterostomes forms the mesoderm and the coelom within

Protocoel

- The anterior coelomic compartment of many deuterostomate or lophophorate animals

- The proboscis

- Forms the protosome

Mesocoel

- The mid-body coelomic compartment of most deuterostomes or lophophorates

- Mouth-bearing collar

- Paired to form the mesosome

Metacoel

- The posterior most coelomic compartment in deuterostomes or lophophorates

- Trunk with the gut and gonads, and the tail

- Paired to form the metasome

Hemichordata

- Marine animals

- Some share similarities with chordates

- Body is tripartite

- Three parts

- Each has 1 or 2 coelomic cavities

- Enteropneusts

- Pterobranchs

Enteropneusts

- Benthic inhabitants of soft sea bottoms

- Acorn worms - free moving deposit and suspension feeders

- Proboscis

- Gland cells on proboscis to produce mucus

- Cilia to move food into the mouth

- Many pairs of dorsal pharyngeal gill slits

- Form pouches in endoderm of the pharynx

- Develop into slits that open to the outside

- Involved in gas exchange and suspension feeding

- Use peristaltic movements for burrowing in sand

- Muscles work against hydrostatic pressure in the tripartite coelom

Pterobranchs

- Tube-dwelling, colonial animals

- Tubes

- Cuticular, secreted by oral shield

- Body is tripartite

- Protosome forms an oral shield

- Protosome carries cilia for movement

- Mesosome forms collar with arms and tentacles used for suspension feeding and gas exchange

- Metasome forms trunk and stalk

Echinoderms

- Invertebrates with an internal skeleton and a system of fluid-filled tubes called a water vascular system

- Adults possess pentamerous radial symmetry

- Adults live on the sea floor

- Larvae possess bilateral symmetry

- Larvae have cilia for swimming

Echinoderm endoskeleton

- Internal skeleton of ossicles which are hard calcareous plates composed of calcium carbonate crystals

- Pores in endoskeleton allow extension of tube feet

Echinoderm larvae

- Endoskeleton already develops at larval stages in some echinoderms (sea urchins)

- Very similar to those of hemichordates

- Non-yolky eggs - larvae feed on plankton

- Both use cilia for locomotion

- Tripartite coelom: protocoel, mesocoel and metacoel

Remodelling of the tripartite coelom in echinoderms

- Metacoels fuse and form a ring, the perivisceral coelom and the genital ring canal

- The left mesocoel forms unique water vascular system

- System of liquid-filled tubes

- Central ring in central disk

- Radial canals that go into 5 arms

- Lateral canals that connect the radial canal to tube feet

- Tube feet:

- Consist of ampulla and podium (foot)

- Often have suckers that secrete sticky chemicals

- Lie on the oral side in the furrow called the ambulacral groove

Ambulacral groove

- A channel along the oral surface of echinoderms through which the tube feet protrude

- Ambulacra form the floor of the groove

- Movable spines may guard the groove

Madreporite (sieve plate)

- Connects to the water vascular system and regulates the amount of seawater in the system

- Osmotic influx of water via madreporite and ciliated stone canal

- Water pushed out of the tube feet, operated by hydraulic power

Roles of the tube feet

- Locomotion

- Action of muscles

- Adhesive chemicals

- Adhesion

- Prey manipulation

- Gas exchange

Gas exchange in echinoderms

- Tube feet

- Located orally

- Water vascular system

- Papillae

- Aboral body wall evaginations

- Formed by the epidermis and the lining of the perivisceral coelom

Echinoderm nervous system

- Nerve ring and radial nerves

- It is not centralised

Echinoidea

- Sea urchins

- Ossicles fused to form rigid skeleton

- Have no arms

- Have 5 rows of tube feet for:

- Slow locomotion

- Chemosensing

- Food manipulation

- Possess mobile spines

- Assist in locomotion and food manipulation

- Possess pedicellariae (pincers)

- Have jaw-like structures around their mouth for cutting and eating seaweed

What differs between the locomotion of echinoderms and hemichordates and the locomotion of chordates?

- The larvae of echinoderms and hemichordates use beating bands of cilia in locomotion

- Chordates use lateral body undulations: bilateral segmented muscles, notochord, medial fins, muscular postanal tail

Newton's First Law of Motion

- Law of inertia

- A body at rest remains at rest unless acted upon by an external force

- Likewise a body in motion will remain in motion unless acted upon by an external force

Newton's Second Law of Motion

The rate of change of the momentum of a body is directly proportional to the net force acting on it, and the direction of the change in momentum taking place in the direction of the net force

Newton's Third Law of Motion

- Law of action and reaction

- Whenever a body exerts a force on another body, the latter exerts a force of equal magnitude and opposite direction on the former

- Thus, in order to move, animals exert a force on their environment

- In return, the environment exerts a force onto them with an equal magnitude but in the opposite direction

- This force propels the animal forward

- Fish use their caudal (posterior) fin to push water sideways and backwards

- Water exerts a force of the same magnitude, but in the opposite direction of the fish

What two structures are needed to exert force?

- Muscles

- Skeletal elements to work against the environment (the vertebral column)

Notochord

- A longitudinal supporting rod of fluid-filled cells that runs through a chordate's body just below the nerve cord, in the embryonic midline

- Present in basal chordates and in embryos/larvae of all chordates

- Can be surrounded by fibrous and elastic sheaths

- Acts as a hydrostatic skeleton

- Incompressible

- Flexible

Muscle arrangement in chordates

- In bilateral segments

- Allows chordates to swim through undulating movements of the body

- Bilateral segments allow for alternating left-right muscle contractions and waves of contractions going anterior to posterior

Undulations

- The notochord is not compressible but it can bend

- When one muscle segments contracts (unilaterally), the anteroposterior axis is not shortened but the notochord and body axis bend, while the somites on the other muscle segment relaxes

How is force increased whilst swimming?

- The surface area that pushes the water can be optimised:

- By having a post-anal tail and medial fins

- More water can be pushed aside

- More thrust generated

- Acceleration of water can be optimised

- By exerting powerful strokes - powerful muscles

- By increasing beat frequency

Dorsal nerve cord

- A hollow tube above the notochord

- Forms from the plate of dorsal ectoderm

- Develops into the CNS

- Brain anteriorly

- Spinal cord posteriorly

Pharyngeal slits

- Present in hemichordates and chordates

- Form from:

- Pouches in endoderm of pharynx

- Develop into slits that open to outside

- Allow water to leave the body without passing through the entire digestive tract

- Suspension feeding device

- Gas exchange

- Optimised to gills in fish

Chordata

- Date back to the Mid-Cambrian period

- Lived in oceans

- Earliest chordates had a notochord, segmented muscles, a tail fin and pharyngeal slits

Cephalochordata

- Notochord reaches up to the anterior tip - in vertebrates the notochord ends midway through the head

- Poorly developed head

- Blade shape, sharp at both ends

- Lancelets

Cephalochordata larvae

- Possess all chordate characteristics:

- Notochord

- Bilateral segmented muscles

- Post-anal tail

- Medial fins

- Dorsal nerve cord

- Pharynx with pharyngeal slits and an endostyle

- Swim freely and feed on plankton

Cephalochordata adults

- Retain all chordate characteristics

- Live in shallow water and often burrow

- Notochord can be stiffened

- Mouth usually sticks out of gravel

- Suspension feeders

- Pharyngeal arches covered with mucus to filter food

- Food passed onto the intestine

- Water moves through the pharyngeal slits via the atrium - external body cavity formed by the outgrowth of the body wall

- Gas exchange via body surface

Hox genes in lancelets

Hox gene expression reveals that genes involved in the regionalisation of the vertebrate brain is already expressed in a similar pattern in lancelets - homologous to the vertebrate brain

Urochordata

- Notochord present in the tail of larva and does not reach its anterior tip

- Tunicata

- Made from cellulose like tunicin

- Marine species

- Solitary forms and colonies

- Most common are sea squirts

- Very specialised suspension feeders - pharyngeal slits and endostyle that produces mucus (traps food)

Urochordata larvae

- Fairly typical chordate

- Once it settles on its substrate it undergoes radical metamorphosis

- Resorbs/degenerates the notochord, its muscular tail and dorsal nerve cord

- Pharynx with pharyngeal slits and endostyle massively enlarged

Endostyle

In chordates, it secrets mucus that traps particles, and is a precursor to the thyroid gland

Position of adult organs in chordates

- The position of adult organs is inverted in chordates along the dorsoventral (D/V) axis

- Arthropods have a ventral nerve cord while chordates have dorsal nerve cord

- Arthropods have a dorsal heart while a chordate's heart is located ventrally

- Position of heart in hemichordates is dorsal

- Suggests that an inversion happened as chordates evolved from the deuterostome ancestor

Ancestral features of deuterostomes

- Radial cleavage

- Regulative embryos

- Blastopore forms the anus

- Enterocoelous coelom (folds of the archenteron form the coelom)

- Pharyngeal slits

Derived features of echinoderms and hemichordates

- Tripartite coelom

- Similar non-yolky larvae that use bands of cilia

Derived features of chordates

- Notochord

- Segmented muscle

- Medial fins

- Postanal tail

- Dorsal hollow nerve chord (CNS)

- Inverted dorsoventral axis