Ascidian Tadpole -

Pip did not eat. He had no mouth and no stomach. He was a living battery, powered only by the energy his mother had packed into his cells, and he knew—in the way a cluster of neurons can "know"—that time was running out.

The is a tiny, free-swimming larva that represents a fleeting moment of mobility in the life of a sea squirt. Though it measures only about 1 mm and lives for just a few days, it possesses complex features—like a primitive spinal cord (notochord) and a simple brain—that it will eventually digest to become a stationary adult. The Great Descent of Pip ascidian tadpole

A "light-eye" that told him to swim away from the bright surface where predators lurked. Pip did not eat