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13. Simple life
| 13.19 |
Level III life: Worms
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| Worm (animal), any soft-bodied, usually small and often elongated animal lacking well-developed limbs. The term does not refer to any particular animal group, but is applied to many unrelated invertebrates or their larvae and to a few vertebrates. The major groups are discussed here. | ||
| The familiar earthworm burrows in soil and feeds on dead materials, extracting organic matter from the soil. This moderately complex animal has a complete digestive tract and a circulatory system. | ||
| The flatworms are simpler animals that lack an intestine, an anus, and a circulatory system. Some flatworms are free-living and occur in the sea, fresh water, and moist land areas. A familiar example is the freshwater planarian, which crawls about and feeds on small animals. Others, such as the tapeworm and fluke, live inside other animals. The bodies of these parasites tend to be degenerate, or simpler in form than their free-living relatives, in all parts except the reproductive system. | ||
| The roundworms are inconspicuous but common. Many roundworms-the pinworm and the hookworm, for example-are parasites of plants and animals; some cause major health problems such as trichinosis and elephantiasis. | ||
| Many worms occur only in the sea. The acorn worm has features that suggest a relationship to the chordate lineage. The arrow worm is a peculiar creature that feeds on small animals in the open water and is often abundant. The peanut worm is a distant cousin of the earthworm, and the ribbon worm is related to flatworms. Tube worms belong to various groups and often feed with tentacles. | ||
| Scientific classification: Earthworms belong to the phylum Annelida. Flatworms make up the phylum Platyhelminthes and roundworms the phylum Nematoda. | ||
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