Concepts of Biology · Diversity of Animals
Echinoderms and Chordates
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In 30 seconds
Echinoderms and chordates seem an unlikely pair. Sea stars, sea urchins, and sea cucumbers (echinoderms) are slow-moving marine animals with five-fold symmetry, while chordates include fish, birds, and humans. Yet both groups are deuterostomes — animals whose embryos develop the anus before the mouth — and that shared developmental heritage places them on the same branch of the animal family tree. Echinoderms are the closest major invertebrate relatives of chordates, so understanding a sea star helps you understand your own body plan.
The pair illustrates two big evolutionary ideas. First, body symmetry can change over a lifetime: echinoderm larvae are bilaterally symmetrical, but adults are usually radially symmetrical (often five-part, or pentaradial). Second, a few defining features can unite a staggeringly diverse group: all chordates, from a sea squirt to a human, share the same four hallmark features at some stage of life.
Why this matters
- We are chordates: the chordate body plan (Notochord Flexible dorsal support rod in chordate embryos Full entry →, Dorsal hollow nerve cord Nerve tube along the animal's back Full entry →, Pharyngeal slits Openings in the chordate throat region Full entry →, post-anal tail) explains the fundamental architecture of the human body — including the origin of the vertebral column and spinal cord.
- Ecology and coastal economies: sea stars are keystone predators in intertidal communities; sea urchins shape kelp forests; sea cucumbers are harvested for food and traditional medicine.
- Biomedical research: sea urchin embryos are classic models for studying fertilization and development, and echinoderms' ability to regenerate lost parts makes them model organisms for tissue repair.
- Exam logic: Deuterostome Animal whose embryo forms the anus before the mouth Full entry → vs. protostome development and the chordate "four features" list are among the most tested topics in introductory biology.
The college version
Core Concepts
Deuterostome development
In deuterostomes, the embryo's first opening (the Blastopore The embryo's first opening Full entry →) becomes the anus, and the mouth forms later. Cell division (cleavage) is radial — cells stack in a regular pattern — and early cells have somewhat flexible fates (regulative development). Protostomes (mollusks, annelids, arthropods) show the opposite: mouth-first, spiral cleavage. Supported by molecular phylogenetics, this split divides the bilaterally symmetrical animals into two great lineages; deuterostomes include echinoderms, hemichordates, and chordates.
The echinoderm body plan
Echinoderms (Greek for "spiny-skinned") are marine animals with:
- Endoskeleton of Ossicles Calcium carbonate plates in echinoderm skin Full entry →: calcium carbonate plates embedded in the skin, often bearing spines. In sea urchins the plates fuse into a rigid test; in sea stars they form a flexible framework.
- Pentaradial symmetry Adult body organized in five parts Full entry →: adults are organized in five (or multiples of five) parts around a central axis — five arms in sea stars, five rows of Tube feet Small suction-cup appendages moved by water pressure Full entry → in urchins. Larvae, however, are bilaterally symmetrical, showing that radial symmetry is a derived adult trait.
- Water vascular system Seawater-filled canals powering tube feet Full entry →: a network of seawater-filled canals that powers tube feet. By changing water pressure, the animal extends and retracts hundreds of tiny suction-cup feet for slow movement, gripping, and opening prey. Seawater enters through a sievelike Madreporite Sievelike plate admitting seawater into the water vascular system Full entry →.
- No brain: a nerve ring and radial nerves coordinate the body.
Major classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Echinoidea (sea urchins and sand dollars), Holothuroidea (sea cucumbers), and Crinoidea (sea lilies). Sea stars are predators that can evert their stomach through their mouth to digest prey externally, and many can regenerate lost arms.
The chordate body plan: four hallmark features
All chordates share four features at least at some stage of development:
- Notochord: a flexible rod of cells along the dorsal side that provides support. In vertebrates it is largely replaced by the vertebral column, but remnants persist (the nucleus pulposus of intervertebral disks).
- Dorsal hollow nerve cord: a nerve tube running along the back; in vertebrates it becomes the spinal cord and brain.
- Pharyngeal slits: openings in the throat region. In invertebrate chordates and fish they filter food or exchange gases; in land vertebrates they appear as transient embryonic structures that give rise to parts of the ear, jaw, and other structures.
- Post-anal tail: a tail extending beyond the anus, used for locomotion in aquatic forms.
Invertebrate chordates
Chordata has three subphyla, two of which are invertebrates:
- Urochordata (tunicates or sea squirts): adults are sessile filter feeders enclosed in a tunic; they show chordate features mainly in the free-swimming larval stage, then lose the notochord and tail as adults.
- Cephalochordata (lancelets): small, fishlike filter feeders that keep all four chordate features throughout life; often used as living models of an early chordate.
Vertebrates: chordates with a backbone
The third subphylum, Vertebrata, is defined by the replacement of the notochord with a vertebral column (backbone) and the presence of a cranium (skull). Vertebrates also have a well-developed head with sensory organs and, in most, a closed circulatory system with a chambered heart. The key point: every vertebrate is a chordate, but not every chordate is a vertebrate.
Common Confusions
| Do Not Confuse | With | Difference |
|---|---|---|
| Echinoderms being protostomes | Echinoderms as deuterostomes | Echinoderms form the anus first, like chordates, not like mollusks/annelids |
| Radial symmetry meaning "no symmetry at all" | Pentaradial symmetry | Echinoderms are organized in five parts around a central axis — still a definite pattern |
| Echinoderms having a brain | Echinoderms having a nerve ring | No brain: a nerve ring plus radial nerves coordinate the body |
| Tube feet being legs | Tube feet as water-pressure appendages | They work by hydraulic pressure from the water vascular system, not muscles alone |
| All chordates being vertebrates | Vertebrates being one chordate subphylum | Tunicates and lancelets are chordates without backbones |
| Adult tunicates showing all chordate features | Tunicate larvae showing them | Adult sea squirts lose the notochord and tail; the larva is the chordate-looking stage |
| The notochord disappearing in humans | The notochord being replaced | It is largely replaced by the vertebral column but persists as the nucleus pulposus |
| "Anus forms first" applying to protostomes | Blastopore → anus in deuterostomes | Deuterostome = "second mouth"; protostome = "first mouth" |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Sea stars and people are distant cousins because both started from an embryo that made its rear end first — a secret family handshake shared by deuterostomes. A sea star is like a slow robot with hundreds of little water-powered suction feet, and its five arms grow from a body built in fives. Every chordate, including you, once had a stiff support rod down its back, a nerve tube, throat openings, and a tail — you just traded the rod for a backbone as you grew.
Worked example
Picture a rocky tide pool. A sea star (an echinoderm) creeps toward a mussel, wraps its arms around the shell, and attaches hundreds of tube feet, pulling steadily until the shell opens a crack. Then it everts its stomach through its mouth, pours digestive juices into the mussel, and absorbs the meal — all without a brain, coordinating through its nerve ring and radial nerves. If a gull tears off one arm, the sea star can regenerate it over the following months.
Now picture a sea squirt (a tunicate chordate) attached to the same rock. As an adult it is a barrel-shaped filter feeder with siphons, and it looks nothing like a chordate. But its free-swimming larva is a tiny tadpole with all four chordate features: a notochord stiffening the tail, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail powering swimming. When the larva settles and attaches, it digests its own tail and notochord — adulthood comes at the price of its chordate costume.
These animals could hardly look more different, yet developmental and molecular evidence places them side by side as deuterostomes. That is the power of looking past adult appearances to shared embryonic history.
Key takeaways
- Echinoderms and chordates are deuterostomes: the blastopore becomes the anus; cleavage is radial.
- Echinoderms: marine only, endoskeleton of ossicles, pentaradial symmetry in adults but bilateral larvae, water vascular system with tube feet, no centralized brain.
- Echinoderm classes: Asteroidea, Ophiuroidea, Echinoidea, Holothuroidea, Crinoidea.
- Sea stars can evert their stomach to digest prey externally and can regenerate arms.
- Chordate hallmark features: notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail — present at some life stage in all chordates.
- Invertebrate chordates: Urochordata (tunicates) and Cephalochordata (lancelets).
- Vertebrata = chordates with a vertebral column + cranium; every vertebrate is a chordate, but not vice versa.
- In humans, the notochord persists as the nucleus pulposus of intervertebral disks.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
What does it mean for an animal to be a deuterostome, and how does that differ from protostome development?
Show answer
In deuterostomes the blastopore becomes the anus and the mouth forms later; cleavage is radial and development regulative. Protostomes form the mouth first with spiral cleavage.
List the four hallmark features of all chordates, and state what each becomes or does in humans.
Show answer
Notochord (support rod; replaced by the vertebral column, persisting as the nucleus pulposus), dorsal hollow nerve cord (becomes spinal cord and brain), pharyngeal slits (filter feeding/gas exchange in aquatic forms; remodeled into ear/jaw structures in land vertebrates), post-anal tail (locomotion in aquatic forms; transient in many embryos).
Why is a sea star's adult symmetry different from its larval symmetry?
Show answer
Echinoderm larvae are bilaterally symmetrical like other deuterostome larvae; adults became radially symmetrical, likely related to their slow or sessile lifestyle. Radial symmetry is a derived adult trait.
How does the water vascular system move a sea star?
Show answer
Seawater enters through the madreporite, travels through the canal system, and is pumped into tube feet; changing pressure extends and retracts the feet, letting the animal grip and pull itself along.
Name two invertebrate chordate subphyla and explain how each shows chordate features.
Show answer
Urochordata (tunicates/sea squirts) show chordate features mainly in the larval stage; Cephalochordata (lancelets) keep all four features as adults.
A student says, "Sea squirts can't be chordates — adults look like bags." How would you correct them?
Show answer
Chordate status is defined by shared features at some stage of life, not adult appearance. The sea squirt larva has a notochord, dorsal hollow nerve cord, pharyngeal slits, and post-anal tail; the adult loses some but is classified by ancestry and development.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Deuterostome
- Animal whose embryo forms the anus before the mouth
- Blastopore
- The embryo's first opening
- Ossicles
- Calcium carbonate plates in echinoderm skin
- Pentaradial symmetry
- Adult body organized in five parts
- Water vascular system
- Seawater-filled canals powering tube feet
- Tube feet
- Small suction-cup appendages moved by water pressure
- Madreporite
- Sievelike plate admitting seawater into the water vascular system
- Notochord
- Flexible dorsal support rod in chordate embryos
- Dorsal hollow nerve cord
- Nerve tube along the animal's back
- Pharyngeal slits
- Openings in the chordate throat region
Sources & references
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