Unit 04 · Echinoderms & the Chordate Transition
This is the unit where the invertebrate story turns toward us. Echinoderms — sea stars, urchins, sand dollars — run their whole lives on a water-vascular system, walking and feeding on hundreds of tube feet, and they develop as deuterostomes, the same early pattern our own lineage follows. From there the unit meets the four traits that define every chordate — a notochord, a dorsal hollow nerve cord, pharyngeal slits, and a post-anal tail — and the invertebrate chordates, lancelets and tunicates, that carry those traits without a backbone. Mastery means you can trace the body-plan clues that link a sea star to a lancelet to a fish.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Water-vascular system & tube feet | Cannot say how a sea star moves or feeds. | Names tube feet but not the system that drives them. | Explains how the water-vascular system powers hundreds of tube feet for movement and feeding. |
| Deuterostome development | Sees no link between echinoderms and chordates. | Knows they share a group but not why. | Explains that echinoderms and chordates are deuterostomes and why that shared early development groups them. |
| The four chordate hallmarks | Cannot name a trait that defines a chordate. | Names one or two hallmarks but leaves the set incomplete. | Names all four — notochord, dorsal hollow nerve cord, pharyngeal slits, post-anal tail — and finds them on a specimen. |
| Invertebrate chordates | Assumes every chordate is a vertebrate. | Knows lancelets or tunicates exist but not why they count as chordates. | Explains that lancelets and tunicates are chordates without a backbone and shows which hallmarks they keep. |
| Lab technique (sea-star & lancelet observation) | Skips the observation or records no structures. | Observes the specimen but misidentifies a key structure. | Observes a sea star and a lancelet, sketches each, and labels the water-vascular structures and chordate hallmarks it can see. |
| Integration (cross-domain) | Treats the science as isolated facts; makes no cross-domain connection. | Names a link to history, reading, or writing but cannot defend why it matters. | Connects the unit to its anchor across History · Reading · Writing (plus chosen electives) and defends why the connection matters. |
“A sea star is an echinoderm — it walks on tube feet run by a water-vascular system, and it develops as a deuterostome, just like we do. A lancelet is a chordate but has no backbone: I can still see the notochord, the nerve cord above it, the pharyngeal slits, and the tail past the anus. Those four traits are what link it forward to the fish.”
“A sea star just crawls somehow. A chordate is an animal with a spine. Lancelets aren’t really related to us, are they?”
You demonstrate this unit through sea-star and lancelet observation where you locate the water-vascular structures and the four chordate hallmarks and explain them aloud — not a multiple-choice test. A criterion counts as mastered only when you can both find the structures and explain how they link echinoderms to the chordates. Mastery is demonstrated, not awarded.
A 5-page clipboard packet — unit overview, key terms, the mastery rubric, anchor examples, and a score sheet you can print and grade against.