Getting Organized

Animals are classified into various groups on the basis of having similar features. Sharing these similar features suggests that they share a common ancestor or history. The more features they share, the closer the relationship. Arthropods are grouped in the Phylum Arthropoda (AR-thruh-PO-duh) because they all have the following special features: exoskeletons, segmented bodies, pairs of jointed appendages, open circulatory systems, and a ventral nerve cord that runs down the underside of the

Insect respiratory system. Oxygen and carbon dioxide move through a system of tubes (trachea) that branch to all parts of the body. Air enters via the spiracles on the insects' bodies. (Illustration by Wendy Baker. Reproduced by permission.)

animal. Arthropods are further divided into three smaller groups, or subphyla (sub-FAI-leh).

The subphylum Cheliceriformes includes sea spiders, horseshoe crabs, scorpions, spiders, ticks, mites, and their relatives. Their bodies are divided into two major regions, the forebody, sometimes called the cephalothorax (SEH-fe-lo-THO-raeks), and abdomen. The forebody has six pairs of appendages, including the pinchers or grasping arms, claw-like pedipalps (PEH-dih-paelps), and eight walking legs. They never have antennae. The reproductive organs are located at the front or rear of the abdomen. The abdomen sometimes has a tail-like structure that is used as a rudder (horseshoe crabs), a defensive weapon (scorpions), as a sensory organ (whip scorpions), or a silk-producing organ (spiders and mites). There are about sixty-one thousand species, most of which live on land.

The Uniramia includes arthropods with only one pair of antennae and legs that are not branched at their bases. Insects and their relatives have bodies that are divided into three major regions: the head, thorax, and abdomen. The head has five pairs of appendages, including the mouthparts and one pair of antennae. Adults have three pairs of legs and sometimes one or two pairs of wings. Their reproductive organs are located toward the rear of the abdomen.

Centipedes, millipedes, and their relatives have bodies that are divided into two major regions. The head is followed by a long trunk-like body. The head has four pairs of appendages, including the mouthparts and one pair of antennae. Adults have one or two pairs of legs on most body segments. Depending on the species the adults have eleven to 382 pairs of legs. Their reproductive organs are located at the end of the body or just behind the head. There are about 818,000 species of insects, millipedes, centipedes, and their relatives that live on land or in freshwater habitats.

The Crustacea include crabs, lobsters, crayfish, shrimp, barnacles, beach hoppers, pillbugs, and their relatives. Their bodies are divided into two major regions: the head, which is usually covered by a broad shield, or carapace (KARE-a-pays), and the body trunk. The head has five pairs of appendages, including the mouthparts and two pairs of antennae. The appendages of crustaceans are usually branched at their bases. The abdomen may also have paired appendages underneath. The reproductive organs are usually found on the midsection or near the front of the abdomen. There are about sixty-seven thousand species, most of which live in the ocean.

Classifications help scientists sort and identify species, as well as organize and locate information about them. But the system of classification is not carved in stone. As understanding of these animals continues to improve, classifications will also change. Groups will be combined, divided, added, or discarded. This constant state of change is sometimes frustrating, but the goal is to have a classification that reflects the true relationships of all organisms based on their evolutionary history.

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