Arthropods

Arthropods

Arthropods
Reading Level

	edHelper's suggested reading level:	grades 6 to 8
	Flesch-Kincaid grade level:	7.51

Vocabulary

	challenging words:		foul-smelling, microscopic, metamorphosis, abdomen, internal, invertebrate, external, directly, segment, krill, arthropod, lead, member, waterproof, during, drying
	content words:		In Greek

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Arthropods
By Cindy Grigg

¹     An arthropod (phylum Arthropoda) is an invertebrate animal that has an external skeleton, a segmented body, and jointed appendages. Wings, mouthparts, and legs are all appendages. The arthropod phylum is named for the jointed legs that each member of the phylum has. In Greek, arthro means "joint," and pod means "foot" or "leg."

²     Insects and spiders are probably the most familiar arthropods to us, but the phylum also includes animals like crabs, lobsters, centipedes, and scorpions. There are more species of arthropods on Earth than of all other animals combined. More than 875,000 species have been identified.

³     Arthropods have open circulatory systems. The blood leaves the blood vessels and bathes the internal organs. It is not enclosed inside blood vessels throughout the body. All arthropods have an external skeleton- an exoskeleton. Some exoskeletons are hard, like that of a crab. It is made out of chitin. Some exoskeletons are leathery, like that of a caterpillar. Both types are waterproof. The exoskeleton keeps the animal from drying out. Arthropods were the first animals to move out of water and onto land, and their exoskeletons probably enabled them to do this.

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