![]() ![]() The open circulatory system distributes blood from the heart to an enlarged blood space by the use of arteries. They do not possess jaws (mandibles), but instead have cutting or piercing appendages called chelicerae. The mouth of arachnids is not readily noticeable from the external surface. Abdominal appendages are either lacking, or modified into special organs such as the spinnerets of spiders and the pectines of scorpions.Īrachnids breathe by means of tracheae (windpipes), book lungs (modified gills), or both. The abdomen may be segmented (as in scorpions) or unsegmented (as in most ticks and spiders). The 12-segment posterior (rear) part of the body, the abdomen (or opisthosoma), contains the genital opening and other structures. The other limb pairs are used for walking. The first pair of limbs (chelicerae) attached in front of the mouth may form pincers or poison fangs, and the second pair (pedipalps) behind the chelicerae may serve as pincers, feelers, or additional legs. A carapace-like shield completely or partially covers the cephalothorax of arachnids. The cephalothorax is composed of an anterior, unsegmented region called the acron, and six true segments (each bearing a pair of appendages). The anterior (front) part, called the cephalothorax (or prosoma), contains sense organs, mouthparts, and limbs or appendages in pairs. The 18-segment arachnid body is often protected by sternites below and tergites above, connected by a soft pleural membrane, and is divided into two tagmata: anterior and posterior.
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