Bio07_TR__U08_CH28.QXD



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Summary

28-1 Introduction to the 28-2 Groups of Arthropods

Arthropods Arthropods are classified based on the

Phylum Arthropoda includes animals such number and structure of their body seg-

as crabs, spiders, and insects. Arthropods ments and appendages—particularly their

have a segmented body, a tough exoskele- mouthparts.

ton, and jointed appendages. An exoskele- Crustaceans—subphylum Crustacea-

ton is an external body covering. An include crabs, shrimps, lobsters, crayfishes,

arthropod exoskeleton is made from protein and barnacles. Crustaceans typically have

and a carbohydrate called chitin. All arthro- two pairs of antennae, two or three body sec-

pods have jointed appendages. Appendages tions, and chewing mouthparts called

are structures such as legs and antennae mandibles. Crustaceans with three body sec-

that extend from the body wall. tions have a head, a thorax, and an abdomen.

The evolution of arthropods—by natural The thorax lies just behind the head and

selection and other processes—has led to houses most of the internal organs. In crus-

fewer body segments and highly special- taceans with two sections, the head and tho-

ized appendages for feeding, movement, rax are fused, forming a cephalothorax.

and other functions. Most living arthropods Chelicerates—subphylum Chelicerata—

have only two or three segments. Living include horseshoe crabs, spiders, ticks, and

arthropods have specialized appendages scorpions. Chelicerates have mouthparts

such as antennae, walking legs, wings, and called chelicerae and two body sections.

mouthparts. Nearly all chelicerates have four pairs of

Arthropods include herbivores, carni- walking legs. Chelicerates are divided into

vores, and omnivores. Most terrestrial two main classes—Merostomata and Arach-

arthropods breathe through a network of nida. Class Merostomata includes horse-

branching tracheal tubes that extend shoe crabs. Horseshoe crabs are the oldest

throughout the body. Air enters and leaves living arthropods. Class Arachnida includes

the tracheal tubes through small openings spiders, mites, ticks, and scorpions. Spiders

called spiracles. Other terrestrial arthro- are the largest group of arachnids. Spiders

pods, such as spiders, respire using book spin strong webs by forcing liquid silk

lungs. Most aquatic arthropods have gills. through spinnerets, organs that contain silk

Arthropods have an open circulatory sys- glands.

tem. Most terrestrial arthropods dispose of Uniramians—subphylum Uniramia—

nitrogen-containing wastes using saclike include centipedes, millipedes, and insects.

organs called Malpighian tubules. Terres- Uniramians have jaws, one pair of anten-

trial arthropods have internal fertilization. nae, and unbranched appendages. Cen-

Aquatic arthropods have internal or exter- tipedes have a few to more than 100 pairs of

nal fertilization. legs. Most body segments have one pair of

When arthropods outgrow their legs each. Centipedes are carnivores. Milli-

exoskeltons, they undergo periods of molt- pedes have two, not one, pairs of legs per

ing. During molting, an arthropod sheds its segment. Millipedes feed on dead or decay-

entire exoskeleton and manufactures a ing plant material.

larger one to take its place.

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107

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28-3 Insects 28-4 Echinoderms

Insects have a body divided into three Phylum Echinodermata consists of animals

parts—head, thorax, and abdomen. Three such as sea stars, sea urchins, and sand

pairs of legs are attached to the thorax. A dollars. Echinoderms are characterized by

typical insect has a pair of antennae, a pair spiny skin, a water vascular system, and

of compound eyes, and two pairs of wings. suction-cuplike structures called tube feet.

Compound eyes are made of many lenses, Echinoderms have an endoskeleton, which

and they detect minute changes in color and is an internal skeleton. Most adult echino-

movement. derms exhibit five-part radial symmetry.

Insects have three pairs of appendages Echinoderm larvae exhibit bilateral sym-

used as mouthparts, including a pair of metry Echinoderms are deuterostomes—an

mandibles. Insect mouthparts are a variety indication that echinoderms and vertebrates

of shapes. are closely related.

The growth and development of insects Echinoderms have a system of internal

usually involve metamorphosis, which is a tubes called a water vascular system. The

process of changing shape and form. In water vascular system is filled with fluid. It

incomplete metamorphosis, the immature carries out many essential body functions in

forms of insects look very much like adults. echinoderms, including respiration, circula-

The immature forms are called nymphs. tion, and movement. It opens to the outside

Nymphs gradually acquire adult structures, through a sievelike structure called a

such as wings, and functional sex organs. madreporite. In sea stars, the madreporite

Insects such as bees, moths, and beetles connects to a ring canal. From the ring

undergo complete metamorphosis. These canal, five radial canals extend along body

insects hatch into larvae that look and act segments. Attached to each radial canal are

nothing like adults. A larva changes into a hundreds of tube feet. A tube foot is a struc-

pupa, the stage in which an insect changes ture that operates much like a suction cup.

from larva to adult. In most echinoderms, waste is released as

Insects are known for their destructive feces through the anus. Most echinoderms

effects. Termites destroy wood, and move using their tube feet. Echinoderms

mosquitoes bite humans. Yet, insects are reproduce by external fertilization.

also beneficial to humans. For example, Classes of echinoderms include sea

insects pollinate many crops. urchins and sand dollars, brittle stars, sea

Insects communicate using sound, cucumbers, sea stars, and sea lilies and

chemical, and other types of signals. feather stars. Echinoderms are common in a

Pheromones are specific chemical messen- variety of marine habitats. Sea urchins help

gers that affect behavior or development in control the distribution of algae and other

other individuals of the same species. forms of marine life. Sea stars are important

Ants, bees, termites, and some of their predators that help control the numbers of

relatives form complex associations called clams and corals.

societies. A society is a group of animals of

the same species that work together for the

benefit of the whole group.

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