Rucete ✏ Campbell Biology In a Nutshell
Unit 5 THE EVOLUTIONARY HISTORY OF BIOLOGICAL DIVERSITY — Concept 33.5 ㅊ
Echinoderms (like sea stars and sea urchins) and chordates (including all vertebrates) belong to the Deuterostomiaclade, characterized by radial cleavage and anus development from the blastopore. Though they seem very different, molecular data show that they are closely related.
What Makes a Deuterostome?
Deuterostomes are bilaterians with:
Radial, indeterminate cleavage
Anus formed from the blastopore
Defined mainly by DNA evidence, not just development.
Includes echinoderms, chordates, and a few other phyla.
Some animals (like brachiopods) show deuterostome development but aren't classified as deuterostomes.
Echinoderms (Phylum Echinodermata)
Marine animals with slow movement or are sessile.
Covered by a thin skin over a hard endoskeleton of calcareous plates.
Unique water vascular system:
Branching hydraulic canals ending in tube feet used for movement and feeding.
Sexual reproduction typically involves external fertilization.
Symmetry:
Larvae: bilateral
Adults: radial (but not perfectly symmetrical—e.g., madreporite is off-center)
Five Clades of Living Echinoderms
Asteroidea (Sea Stars and Sea Daisies)
Sea stars: central disk with arms; move and feed using tube feet.
Feed by everting their stomachs into prey (e.g., clams), releasing digestive enzymes externally.
Can regenerate arms—and in some cases, entire bodies.
Sea daisies: tiny, disk-shaped, no arms; absorb nutrients through body membrane.
Ophiuroidea (Brittle Stars)
Long, flexible arms and distinct central disk.
Move by lashing arms, not tube feet.
Some are scavengers; others are predators or filter feeders.
Echinoidea (Sea Urchins and Sand Dollars)
No arms; have five radially arranged sections of tube feet.
Sea urchins use spines and tube feet for movement.
Have jaw-like structures for grazing on seaweed.
Crinoidea (Sea Lilies and Feather Stars)
Sea lilies: attached by a stalk; feather stars: mobile.
Use upward-facing arms to filter-feed.
Fossils date back 500+ million years.
Holothuroidea (Sea Cucumbers)
Elongated shape, no spines, reduced skeleton.
Use tube feet around the mouth as feeding tentacles.
Retain radial symmetry features in five body sections.
Chordates
Includes lancelets, tunicates, and vertebrates.
Share traits with echinoderms but have evolved independently for over 500 million years.
Bilateral symmetry, segmented bodies, and a coelom.
Full chordate phylogeny covered in Chapter 34.
In a Nutshell
Despite their outward differences, echinoderms and chordates are united by a common evolutionary heritage as deuterostomes. Echinoderms show diverse adaptations like tube feet, external digestion, and radial adult forms, while chordates went on to give rise to vertebrates. Their shared ancestry underscores both the unity and diversity of life.