An acoel is a tiny flatworm without a gut. Denizen of stony shallows, photosynthetic algae in its skin help nourish it.

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An acoel is a tiny flatworm without a gut. Denizen of sto...

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An acoel is a tiny flatworm without a gut. Denizen of stony shallows, photosynthetic algae in its skin help nourish it.

Darlyne A. Murawski/National Geographic Image Collection

Kewalo Basin, Honolulu, Oahu, Hawaii. An acoel flatworm.

An acoel is a tiny flatworm without a gut. Denizen of stony shallows, photosynthetic algae in its skin help nourish it.

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Unfurling marvels, ribbon worms vary from a few inches long to nearly 200 feet (60 meters)the longest animals on the planet. Voracious predators that hide under rocks in intertidal zones or in the open sea, many are equipped with neurotoxin-tipped barbs on their proboscises for impaling other worms, their main prey. One of the toxins found in ribbon worms, anabaseine, shows potential for the treatment of Alzheimer's disease. (This species, Baseodiscus delineatus, had not been seen before in Hawaii.)

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.

Plucked from the gut of a fisherman's catch, this parasitic worm is named after the ono fish that hosts it, ono meaning "good to eat" in Hawaiian. The worm "smelled to high heaven," says photographer Darlyne Murawski. "It was full of blood." Such parasites, called trematodes, take up residence in other animals, feed on and reproduce within them, and then either kill the host or escape. Their bodies are almost entirely dedicated to reproduction.

A net of fine hairs, shown in a close-up of a feather duster worm's tentacles, sweeps particle-laden water to grooved tracks, which carry the particlesincluding food and silt for building its protective tubeto the animal's mouth. Bright colors may warn would-be predators of toxicity.

Spawning on a summer night, fire worms emit clouds of eggs and sperm at high tidea performance triggered by the full moon. Harpoon-like bristles filled with a neurotoxin protect the worms, which inhabit shallow reefs and feed on crustaceans and anemones. Hawaiians call the worm 'aha huluhulu, "hairy cord."

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