Deuterostomes: Structure, Diversity, and Evolution

Deuterostomes

Overview of Deuterostomes

Deuterostomes are a major clade of bilaterian animals distinguished by their embryonic development and anatomical features. This group includes the phyla Echinodermata, Hemichordata, and Chordata.

• Definition: In deuterostomes, the blastopore (first opening in the embryo) becomes the anus, while the mouth forms secondarily.

• Developmental Features:

  • Radial, indeterminate cleavage during early embryogenesis.

  • Coelom forms by outpocketing of the gut (enterocoely).

• Main Phyla: Echinodermata, Hemichordata, Chordata.

Feature	Deuterostomes	Protostomes
Blastopore fate	Anus	Mouth
Cleavage	Radial, indeterminate	Spiral, determinate
Coelom formation	Outpocketing of gut	Splitting of mesoderm

Phylum Echinodermata

General Characteristics

Echinoderms are exclusively marine, benthic animals with a unique body plan and physiology.

• Endoskeleton: Composed of calcareous ossicles.

• Water Vascular System: Specialized system for locomotion, feeding, and gas exchange.

• Symmetry: Adults exhibit pentaradial symmetry; larvae are bilaterally symmetrical.

Echinoderm Water Vascular System

The water vascular system is a network of hydraulic canals unique to echinoderms.

• Functions: Locomotion, feeding, gas exchange.

• Pathway: Water enters through the madreporite → stone canal → ring canal → radial canal → tube feet.

• Tube Feet: Ampullae control extension and retraction for movement and feeding.

Major Classes of Echinoderms

Class Asteroidea – Sea Stars

• Central disk with radiating arms.

• Distinction between oral (mouth) and aboral (opposite) surfaces.

• Tube feet with suckers; madreporite on aboral surface.

• Example: Asterias (common sea star).

Class Echinoidea – Sea Urchins & Sand Dollars

• No arms; body encased in a test made of fused ossicles.

• Presence of spines, tube feet, and pedicellariae (small pincer-like structures).

• Pentaradial symmetry evident in body plan.

• Example: Strongylocentrotus (purple sea urchin).

Class Ophiuroidea – Brittle Stars

• Slender, flexible arms and a distinct central disk.

• Tube feet lack suckers.

• Locomotion primarily by arm movement, not tube feet.

• Example: Ophiothrix (brittle star).

Class Holothuroidea – Sea Cucumbers

• Elongated, leathery body with reduced ossicles.

• Tube feet often modified into tentacles around the mouth.

• Pentaradial symmetry is internal, not externally obvious.

• Example: Holothuria (sea cucumber).

Phylum Hemichordata

Acorn Worms

Hemichordates are marine burrowers sometimes called "half chordates" due to shared features with chordates.

• Possess a dorsal nerve cord and pharyngeal slits.

• Lack a notochord and post-anal tail.

• Example: Balanoglossus (acorn worm).

Phylum Chordata

Defining Characteristics

Chordates are coelomate deuterostomes with four key features present at some stage of development.

1. Notochord: Flexible rod for body support; precursor to vertebral column.

2. Dorsal hollow nerve cord: Develops into the central nervous system (brain and spinal cord).

3. Pharyngeal slits or clefts: Used for filter feeding or respiration.

4. Post-anal tail: Muscular tail extending beyond the anus, used for locomotion.

Subphylum Cephalochordata – Lancelets

• Retain all four chordate traits throughout life.

• Marine filter feeders.

• Segmented muscles (myomeres) for movement.

• Example: Branchiostoma (lancelet).

Subphylum Urochordata – Tunicates & Salps

• Larvae possess all four chordate traits and are free-swimming.

• Adults are sessile filter feeders, retaining only pharyngeal slits.

• Body covered in a cellulose-like "tunic".

• Example: Ciona (sea squirt), Salpa (salp).

Subphylum Vertebrata

• Vertebral column replaces the notochord.

• Development of cranium, brain, and cephalization.

• Endoskeleton of bone or cartilage.

• Neural crest cells contribute to head structures.

Jawless Vertebrates – Cyclostomes

• Includes classes Myxini (hagfish) and Petromyzontida (lampreys).

• No jaws, paired fins, or scales.

• Hagfish produce slime for defense; lampreys have a parasitic sucker mouth.

Jawed Vertebrates – Gnathostomes

• Jaws evolved from pharyngeal arches.

• Enabled active predation and larger body size.

• Includes all other vertebrates.

Class Chondrichthyes – Cartilaginous Fishes

• Includes sharks, rays, and skates.

• Skeleton composed of cartilage.

• Dermal denticles on skin; internal fertilization with claspers in males.

Bony Fishes – Osteichthyes

• Actinopterygii: Ray-finned fishes (most diverse group).

• Sarcopterygii: Lobe-finned fishes (ancestors of tetrapods).

• Bony skeleton, gills covered by operculum, swim bladder for buoyancy.

Transition to Land: Tetrapods

• Lobe-fins evolved into limbs and digits.

• Key adaptations: neck mobility, fusion of pelvic girdle, lungs for air breathing.

• Example: Tiktaalik (transitional fossil).

Class Amphibia

• Includes frogs, salamanders, caecilians.

• First vertebrates to colonize land.

• Moist skin aids in gas exchange.

• Life cycle includes aquatic larvae and terrestrial adults (metamorphosis).

Class Reptilia

• Scaly keratinized skin.

• Amniotic egg allows reproduction on land.

• Ectothermic (except birds).

• Birds evolved from reptilian ancestors (archosaurs).

Birds as Reptiles

• Endothermic reptiles with feathers.

• Lightweight skeleton, no bladder, no teeth.

• Adaptations for powered flight.

Class Mammalia

• Endothermic vertebrates with hair and mammary glands.

• Differentiated teeth for varied diets.

• Large brain and complex behavior.

• Three major groups: monotremes, marsupials, placentals.

Frog Dissection

External Anatomy

• Key features: eyes, tympanic membrane, forelimbs, hindlimbs, digits.

• Adaptations for jumping and swimming.

Internal Anatomy (Female and Male)

• Major organs: heart, lungs, liver, stomach, intestines, kidneys.

• Female: presence of ovaries and oviducts.

• Male: presence of testes and associated ducts.

Summary Table: Major Deuterostome Groups

Phylum/Subphylum	Key Features	Examples
Echinodermata	Pentaradial symmetry, water vascular system, calcareous endoskeleton	Sea stars, sea urchins, brittle stars, sea cucumbers
Hemichordata	Dorsal nerve cord, pharyngeal slits, marine burrowers	Acorn worms
Chordata	Notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail	Lancelets, tunicates, vertebrates
Vertebrata	Vertebral column, cranium, endoskeleton	Fishes, amphibians, reptiles, birds, mammals

Additional info: The notes cover topics relevant to Ch. 32 (Animal Diversity), Ch. 33 (Invertebrates), and Ch. 34 (Vertebrates) of a General Biology course, including anatomical and evolutionary features of major deuterostome groups.