Extreme Body Simplification

Stripped-down parasites can lose almost their whole body plan. Rafflesia, a parasitic plant, has no stems, leaves, or roots. It lives as bare threads inside a jungle vine and surfaces only to bloom, producing the largest single flower on Earth, up to a meter wide, purely to reproduce.

The striking part is how predictable this reduction is. Unrelated parasites lose the same organs in the same order, until distant cousins end up looking eerily alike. The endpoint is always the same: maximum reproduction, minimum everything else.