Characteristics Of Arthropods

The members of the phylum Arthropoda are called arthropods (AHR-thruh-PAHDS). Like annelids, arthropods are segmented animals. In arthropods, however, the body segments bear jointed extensions called appendages, such as legs and antennae. In fact, arthropod means "jointed foot."

Another distinguishing feature of arthropods is their exoskele-ton, which provides protection and support. As Figure 36-1 shows, the arthropod exoskeleton is made up of three layers that are secreted by the epidermis, which lies just beneath the layers. The waxy outer layer is composed of a mixture of protein and lipid. It repels water and helps keep terrestrial arthropods from drying out. The middle layer, which provides the primary protection, is made mainly of protein and chitin, a tough polysaccha-ride. In some arthropods, the middle layer is hardened by the addition of calcium carbonate. The inner layer also contains protein and chitin, but its flexibility at the joints allows arthropods to move freely. Muscles that attach to the inner layer on either side of the joints move the body segments.

Arthropods show a high degree of cephalization. Recall that cephalization is the concentration of sensory and brain structures at the anterior end. A variety of segmented appendages around the mouth serve as sensors and food handlers. Most arthropods have segmented antennae that are specialized for detecting chemicals. Most arthropods also have compound eyes—eyes made of many individual light detectors, each with its own lens. In addition, many arthropods have simpler structures that sense light intensity. These sensory structures on the head send nerve impulses to the brain, which coordinates the animal's actions. As in annelids, impulses travel from the brain along a ventral nerve cord, which links ganglia in the other segments of the body.

All arthropods have open circulatory systems. Recall that in an open circulatory system, the heart pumps circulatory fluid through vessels that empty into spaces in the body.

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