Main->Readings->5th Grade Readings->Invertebrates->Part 4
Vocabulary |
Those moist, long, pink worms in your soil are earthworms. Earthworms eat organic material that is in the soil and live underground most of their lives. They belong to a phylum of invertebrates called Annelida. Annelida are animals with segmented bodies. That is, their bodies are divided into parts, or segments. They have nerves and a digestive system and two body openings. There are about 9,000-12,000 different species on Earth.
An unusual fact about earthworms is that they are hermaphrodites, meaning they are both male and female at the same time. Earthworms breathe through their skins and must stay in moist soil. If they dry out, they will die. Every day, an earthworm passes an amount of soil equal to its body weight through its digestive system.
Another type of annelida is the leech. All leeches have segmented bodies like earthworms, but they do not eat organic material in soil. They feed on blood. Leeches all have 34 body sections, they have suckers at their head and tail ends, and they may be tiny in size or as much as 8 inches long. They live in water and on land. The saliva of a leech contains a substance which keeps blood from clotting, so they can feed as much as they want.
Although flatworms, roundworms, and segmented worms are all called worms, they are not closely related. "Worm" is not a scientific term. It just means something long and skinny without legs. In previous centuries, people called snakes "worms," too, even though snakes are vertebrate animals.
Those star-shaped creatures found at the beach are also animals. Sea stars belong to the phylum of Echinoderms. The creatures which belong to the echinoderm phylum are small, slow, spiny-skinned animals with no brain, often with five-pointed shapes or designs. Other members of the Echinoderms are sea urchins, sea cucumbers, and sand dollars. The Echinoderms are unusual because they have a stiff inside skeleton which supports them. All of them live underwater.
Sea stars feed in an unusual way. They particularly enjoy eating mollusks, especially the bivalves, which have very strong muscles to keep their shells closed. To eat a clam, a sea star will turn its stomach inside out and stick it into the clam, then begin digesting.
Another unusual fact about sea stars is the way they move. They have tiny tube feet on their undersides. They pump water through their bodies into the tube feet, making them stick out. When the tube feet touch a rock, the sea star withdraws the center of the foot, making a suction cup. Sea stars have quite a grip. Like planarians and sponges, sea stars can re-grow if they are cut into pieces. Each part will grow into a new sea star.
The most peculiar fact about spiny-skinned animals is that they are more closely related to human beings than to jelly-fish or octopuses.
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This page last modified August 15, 2002
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