Tube Feet and Tiny Plates: Who Is the Sea Cucumber Related To?

Low Tide at Pelican Rock

1Mr. Lorcan Fairweather's eighth-grade marine biology class visited the tide pools at Pelican Rock on the Pacific coast during a morning low tide. The students worked in pairs with a field guide and a hand lens, and they followed the class rules: look closely, touch gently with one wet finger, and leave every animal where it was found. Priscila Varga and her partner chose five animals to study for their report.

2Ochre sea stars clung to the rocks with five thick arms. Underneath each arm, a groove held rows of tiny, soft tube feet, each one ending in a small sucker. Purple sea urchins sat in shallow holes in the rock. Between their spines, long tube feet stretched out in five double rows that ran from the bottom of the round shell to the top. In a crack lay a California sea cucumber, a soft, reddish-brown animal about as long as a cucumber. Rows of tube feet ran along its body, and a ring of branched tentacles around its mouth, which are changed tube feet, gathered bits of food from the sand.

3A giant green anemone looked like a flower, with rings of sticky tentacles around a central mouth. When Priscila touched a tentacle, it clung to her finger, because the tentacles are packed with stinging cells that are usually too weak to hurt a person's finger. Under a stone in a mussel bed, the pair found a mussel worm, a kind of bristle worm. Its long body was made of many repeating segments, and each segment had a pair of fleshy paddles tipped with bristles.

4Back at school, Mr. Fairweather showed the class skin from each animal under a microscope. Sea star and sea urchin skin held tiny hard plates, and in the urchin the plates were joined into its round shell. The sea cucumber's skin held hard plates too, but each one was smaller than a grain of sand. He also passed around a fossil sea lily about 350 million years old. Sea lilies, which still live in parts of the deep ocean, sit on a stalk made of stacked disc-shaped plates and catch food with their arms. Soft parts such as tube feet were not preserved in the fossil.

5Mr. Fairweather reminded the class that animals can look alike because they live in a similar way, even when their bodies are built differently. Scientists give more weight to detailed structures that match part for part.