Microscopic, nearly clear, and covered in hair, a gastrotrich is one of the smallest aquatic worms, inhabiting the minuscule space between sediment particles. Yet each has a mouth, a complete gut, and both sperm and eggs (it reproduces without mating). This specimen, from the Woods Hole Oceanographic Institute in Massachusetts, may appear full of eggs, but is not. A single egg would take up nearly half its body cavity.