| Feature | Polyp | Medusa |
|---|---|---|
| Body form | Cylindrical, tubular | Bell-shaped (umbrella), disc-like |
| Attachment | Sessile (attached to substrate) | Free-swimming (pelagic) |
| Orientation | Mouth and tentacles at top | Mouth and tentacles hanging downward |
| Mesoglea | Thin layer | Thick, gelatinous layer |
| Reproduction | Mainly asexual (budding) | Mainly sexual (produces gametes) |
| Example | Hydra (only polyp), coral polyps | Aurelia (primarily medusa), jellyfish |
| Nerve net | Simple nerve net | More complex nerve net |
| Nematocysts | Present on tentacles | Present on tentacles |
| Movement | None (attached) | Active swimming by pulsing bell |
Organisms and their forms:
- Hydra: Only polyp form (no medusa stage)
- Aurelia (jellyfish): Primarily medusa; small polyp stage (scyphistoma)
- Obelia: Both polyp (feeding polyp = hydranth + reproductive polyp = gonangium) and medusa (Obelia medusa)
- Physalia: Colonial polyp (siphonophore) with no true medusa — each polyp is specialized
Key concept: The alternation between polyp (asexual) and medusa (sexual) generations is called metagenesis or alternation of generations — a defining feature of many Cnidaria.