Swampworm
Overview
Swampworm's progenitors live in wetlands where they acts as both predators and parasites. It drills into its victim and injects an anti-coagulating saliva which keeps the blood flowing. Then it will drink until it is satisfied or its victim is drained of all bodily fluids. When large prey is unavailable, it will filter plankton and algae from the water for nourishment instead.
The Swampworm is yet another good simplistic design. The solid red eyes, long proboscis, and scorpion-like stinger at the end of its tail are all A-okay.