They saw that the creature begins its self-dissection by contracting its midsection to create a waist, changing its shape from a cigar into an hourglass. To study them, Collins and her team filmed one species, Dugesia japonica, continuously for months. They also prefer to split in the dark and will stop if disturbed. Planarians self-fragment just once a month, and the process is over within minutes. The feat has intrigued people since at least the ninth century, but it is hard to observe. This unusual anatomy is even stranger because it can tolerate bisection. Read: The body’s most embarrassing organ is an evolutionary marvel These lead to a ladder-shaped nervous system of two nerve cords that run down the body and are connected by crosswise rungs. They do have brains of sorts-two clusters of neurons in the head. They get oxygen through diffusion, and lack lungs, gills, hearts, and blood vessels. They release liquid waste through pores on their backs. The mouth lies in the middle of the underside, and doubles as an anus. Their bodies are basket-weaves of muscle and connective tissue, with no internal cavities full of soft organs. There are thousands of species of planarians, and they’re all very different from more familiar worms like earthworms. As the naturalist John Graham Dalyell wrote in 1814, planarians could “almost be called immortal under the edge of the knife.” And if Collins needs more animals quickly, she can do with a scalpel what the worms do with their own muscles. Breeding them is a cinch: Given enough food, planarians will repeatedly double themselves by halving themselves. “It’s just mind-blowing,” Eva-Maria Collins of Swarthmore College, who studies these animals, told me.

And even more miraculously, the tail regrows its head. Within days, the head piece grows a tail. After a few minutes of stretching and ripping, it separates into two halves-a head and a tail.

The planarian begins as a small, flattened, sluglike creature with a spade-shaped head and two googly eyes. Others, more straightforwardly, tear themselves in two. When planarian flatworms want to reproduce, some have sex.

Flat worms flatworms