Download Arthropods

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Mollusks, Arthropods, and Echinoderms
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Section Summary
Arthropods
Guide for Reading
■ What are the four major groups of arthropods and what are their
characteristics?
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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.
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