The phylum Arthropoda contains more than three-fourths of all the different species of animals. It includes crustaceans, spiders, and insects, as well as many smaller groups. Among the most important groups of arthropods are insects, crustaceans, arachnids, centipedes, horseshoe crabs, and millipedes. Insects make up the largest class of arthropods in terms of the number of species.
An arthropod’s exoskeleton, which is jointed to allow movement, is hardened and does not grow with the animal. As a result, it must be periodically shed—a process known as molting—in order to allow growth. The arthropod body is usually divided into head, thorax, and abdomen, although in some groups there may be no clear distinction between the three regions. The head and thorax may be fused, forming a cephalothorax, or the abdomen may be reduced in size. The head typically carries feeding and sensory structures. Each segment of the body usually has a pair of jointed appendages, which are modified for specific functions in different species.
Arthropods have a body cavity (coelom), but it is small and contains only the gonads and excretory organs. The other internal cavities form a hemocoele (blood cavity). The circulatory system is open, with the heart in the upper part of the body. The digestive system consists of a tubular gut that runs from the mouth to the anus. The foregut and hindgut are lined with chitin and are shed during molting. Excretion is through specialized tubes and the anus is at the back end of the body.
Arthropods have a nervous system with a brain and a nerve cord, which has branches called ganglia in each segment. Bristles that are sensitive to touch are also a common feature. Eyes, which may be simple or compound, are usually present. Compound eyes are unique to arthropods and are ideal for detecting motion.
The trilobites are a fossil group of marine arthropods of the class Trilobita that were once extremely numerous but became extinct toward the end of the Paleozoic era, about 230 million years ago.
Most trilobites were less than 4 inches (10 centimeters) long, and the body was divided into a head, or cephalon; a thorax; and a tail, or pygidium. The cephalon was composed of four or five fused segments and carried a pair of antennae, a mouth, a pair of eyes, and four pairs of forked appendages, which functioned as both gills and legs. The segments of the thorax were movable, whereas those of the pygidium were fused to form a solid shield. Each segment of the thorax possessed a pair of walking legs.
Despite their name, horseshoe crabs (order Xiphosura) are not crabs at all, but primitive marine arthropods. They are among the only surviving members of a large group of ancient animals of the class Merostomata.
The horseshoe crab’s body is divided into two parts: the prosoma, which is covered by the shell and includes the head, and the abdomen. On the underside of the prosoma is a mouth, with a pair of pincerlike feeding appendages called chelicerae, and five pairs of walking legs. There are five pairs of abdominal appendages which are modified as leaflike book gills. The book gills keep a constant current of water circulating over the gills and also act as paddles, which move the animal. The horseshoe crab’s nervous system is well developed, but the eyes are not. They can detect movement but cannot form an image.
During mating, the female horseshoe crab carries the male on her back to shore, where she digs several holes and lays from 200 to 1,000 eggs in each. The male then fertilizes the eggs. Newly hatched horseshoe crabs are called tri Io bite larvae because of their similarity to the larvae of that class.
The class Pycnogonida contains about 600 species of carnivorous marine animals known as sea spiders. They feed on corals and sponges and are found in all marine waters. The sea spider’s head bears clawlike structures called chelicerae, which are used for seizing and tearing apart food; a sucking mouth at the end of a tube; and four eyes. Despite their common name pycnogonids bear no real similarity to true spiders. Pycnogonids have a segmented abdomen and legs that end in claws, whereas spiders do not have these characteristics.
Centipedes and millipedes
The animals in the class Chilopoda are commonly known as centipedes, and those in class Diplopoda as millipedes. Millipedes and centipedes are found mainly in damp conditions, such as rotting logs or in leaf litter, because they do not possess a waxy cuticle with which to reduce water loss.
Both groups have separate sexes, and the female lays eggs that are fertilized by the male. Some species lay the eggs in a “nest,” where they are guarded by the female, but others, such as the centipede Lithobius, lay one egg at a time and then leave it. The young resemble the adult and, in some species of centipedes, have the same number of segments. But other young centipedes and all millipedes have fewer body segments than do the adults.
Centipedes are fast-moving carnivores. They have one pair of antennae on the head and two pairs of jaws. The eyes of centipedes may be simple ocelli, or clusters of light-sensitive cells, or modified compound eyes. Centipedes are flattened and divided into a large number of segments. All of these, except for the first, carry a pair of long, slender, walking legs.
Millipedes have a lower “lip” called a gnathochilarium. Many millipedes lack eyes completely, and those that have eyes have only ocelli. Most millipedes are vegetarian scavengers. Their body is also segmented, but is usually cylindrical. The first four trunk segments differ from the others in that the first is legless, and the following three bear only one pair of walking legs. These four segments together are sometimes called the thorax. The abdomen has many segments, which are fused together in pairs called diplosegments. Each diplosegment bears two pairs of short legs, and as a result, a millipede may have as many as 115 pairs of legs. Nevertheless, millipedes are not good runners and, when attacked, they protect themselves by coiling up. Many species also emit a foul-smelling secretion from special stink glands on the sides of the body.