Of the arachnids, spiders (which make up the Order Araneae) are the most diverse. Arachnids have fairly simple eyes that register only changes in light levels. The final four pairs of appendages are walking legs. The second pair, pedipals, have a sensory function, and may include both receptors sensitive to touch and receptors sensitive to chemical changes. The first pair, the chelicerae, is typically adapted for killing and consuming prey. Like other chelicerates, arachnids have six pairs of appendages. The majority are land-based and most are found in fairly warm, dry habitats. There are over 100,000 described species in this class. The class Arachnida includes scorpions, spiders, ticks, and mites. These include a pair of chelicerae, a pair of pedipals, and four pairs of legs. They possess six pairs of unbranched appendages. They are characterized by the presence of two tagmata (fused segments), a cephalothorax (fused head and thorax), and an abdomen. The chelicerates include the horseshoe crabs, scorpions, spiders, ticks, mites, sea spiders, and other related species. The last consists exclusively of extinct forms. These are the Chelicerata, the Crustacea, the Uniramia, and the Trilobita. Major Groups of ArthropodsĪrthropods are divided into four subphyla. While some species exhibit direct development, in which eggs hatch as miniature versions of adults, other species pass through an immature larval stage and undergo a dramatic metamorphosis before reaching adult form. Fertilization usually occurs internally, and most species are egg laying. In most arthropod species, the sexes are separate. A double nerve cord extends backwardsĪlong the ventral surface of the body, and each body segment is associated with its own ganglion, or mass of nerve cells. Arthropods are characterized by a brain as well as a nerve ring around the area of the pharynx, in the oral cavity. The tracheal respiratory system consists of external openings called spiracles that are linked to a system of branched tubules which allow respiratory gases to reach internal tissues. Some species have gills, while others employ tracheae, or book lungs. Gas exchange in the phylum occurs in various ways. Arthropods also have a complete gut with two openings, the mouth and the anus. Arthropods have an open circulatory system (without blood vessels) that consists of a tube that is the heart and an open hemocoel, the coelom of the animal, in which blood pools. Most species have paired compound eyes, and many also have a number of simpler eyes called ocelli. Some arthropods have highly developed sense organs. Arthropod appendages may be either biramous (branched) or uniramous (unbranched). However, in most species some appendages have been modified to form other structures, such as mouthparts, antennae, or reproductive organs. In early, primitive anthropods, each body segment was associated with a single pair of appendages (attachments). Arthropods also have appendages with joints (the word "arthropod" means "jointed feet"). The head, thorax, and abdomen are examples of tagmata. This process of fusion is called tagmosis. However, a number of segments are sometimes fused to form integrated body parts known as tagmata. Molting allows for rapid growth until the newly secreted exoskeleton hardens.Īrthropod bodies are divided into segments. The exoskeleton is incapable of growth, and is molted (shed) repeatedly during the growth of the animal. Its walls provide anchors for the attachment of muscles. The external skeleton offers organisms protection as well as support for the body. In some species, lipids, proteins, and calcium carbonate may also contribute to the exoskeleton. All arthropods possess a stiff exoskeleton (external skeleton) composed primarily of chitin. Characteristics of Arthropodsĭespite the remarkable variety of arthropod species, all share aspects of a single basic body plan. Arthropods are found in virtually every known marine (ocean-based), freshwater, and terrestrial (land-based) ecosystem, and vary tremendously in their habitats, life histories, and dietary preferences. Some of the more well-known arthropods include insects, crustaceans, and spiders, as well as the fossil trilobites. Countless arthropods remain undescribed (not yet named and studied), and the actual number of living species could be as high as ten million or more. This represents approximately three-quarters of all known biological organisms, living or extinct. It includes well over one million described species. The phylum Arthropoda is the largest and most varied in the animal kingdom.
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