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ArthropodsGuide for Reading■ What are the four major groups of arthropods and what are their

characteristics?

■ How do crustaceans, arachnids, and centipedes and millipedes differ?

The major groups of arthropods are crustaceans, arachnids, centipedes and millipedes, and insects. Arthropods are invertebrates that have an external skeleton, a segmented body, and jointed attachments called appendages. Arthropods have bilateral symmetry, an open circulatory system, a digestive system with two openings, and sexual reproduction.

All arthropods have a waxy exoskeleton, or outer skeleton, that protects the animal and helps prevent evaporation of water. An exoskeleton cannot grow with the animal. Arthropods shed their outgrown exoskeletons in a process called molting.

Arthropods have segmented bodies and jointed appendages that give them flexibility and enable them to move. Sometimes the segments are joined into distinct sections, such as a head, midsection, or hind section. They have specialized appendages that perform certain functions. For example, an antenna (plural antennae) is an appendage on the head that contains sense organs.

A crustacean is an arthropod that has two or three body sections, five or more pairs of legs, and two pairs of antennae.

Crustaceans live in watery environments and must have gills. They obtain food in many ways. Some are scavengers that eat dead plants and animals. Some are predators, eating animals they have killed.

Most crustaceans begin their lives as microscopic, swimming larvae. Crustacean larvae develop into adults by metamorphosis, a process in which an animal’s body undergoes dramatic changes in form.

Arachnids are arthropods with two body sections, four pairs of legs, and no antennae. The first body section is a combined head and midsection. The hind section, called the abdomen, contains the arachnid’s reproductive organs and part of its digestive system. Spiders, mites, scorpions, and ticks are all arachnids.

Centipedes and millipedes are arthropods with two body sections and numerous pairs of legs. Centipedes have one pair of legs attached to each segment, and some centipedes have over 100 segments. Millipedes, which may have more than 80 segments, have two pairs of legs on each segment.

Mollusks, Arthropods, and Echinoderms ■ Section Summary