Unit 03 · Mollusks & Arthropods
These are the two most successful invertebrate designs on Earth. Molluscs build a soft body on three parts — a muscular foot, a mantle that secretes the shell, and a visceral mass — and run that plan from snails and clams all the way to the octopus. Arthropods went the other way: a hard exoskeleton they must molt to grow, a segmented body, and jointed limbs, in the four great groups — insects, arachnids, crustaceans, and myriapods. Mastery means you can name the group from its body plan and explain why an exoskeleton and jointed legs let arthropods outnumber every other kind of animal.
| Criterion | Not yet | Approaching | Mastered |
|---|---|---|---|
| Mollusc body plan | Cannot name a shared trait behind snails, clams, and squid. | Names the parts but cannot say what the foot or mantle does. | Identifies the foot, mantle, and visceral mass and explains the job of each across the mollusc classes. |
| Gastropods, bivalves & cephalopods | Treats every mollusc as one kind of animal. | Names the classes but places examples in the wrong one. | Sorts a snail, a clam, and an octopus into the right class from observable traits. |
| Exoskeleton & molting | Thinks an exoskeleton grows with the animal like skin. | Knows arthropods have a hard shell but not why they molt. | Explains that the exoskeleton must be shed and regrown to allow growth, and what that costs the animal. |
| Arthropod groups & jointed limbs | Calls every small animal a bug or an insect. | Names the groups but confuses a spider with an insect. | Uses body segments, leg count, and antennae to tell insects, arachnids, crustaceans, and myriapods apart. |
| Lab technique (dissection & dichotomous key) | Damages the specimen or cannot use the key. | Dissects carefully but keys the animal to the wrong group. | Dissects a crayfish or grasshopper cleanly, names its segments and appendages, and keys it to its group with a dichotomous key. |
| 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. |
“This one has three body parts, six legs, and antennae, so it’s an insect — not the eight-legged, two-part arachnid next to it. Both are arthropods, so both wear an exoskeleton they have to molt to grow. The clam beside them is a mollusc: soft body, muscular foot, and a mantle that built the shell.”
“It’s a bug, and spiders are bugs too. The shell is just its skin. A clam is basically the same as a snail, I think.”
You demonstrate this unit through a crayfish or grasshopper dissection where you name the animal’s segments and appendages and key it to its group with a dichotomous key aloud — not a multiple-choice test. A criterion counts as mastered only when you can both handle the dissection and explain the body plan that places the animal. 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.