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It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects to an abdomen (and is often referred to as such), the opisthosoma is differentiated by its inclusion of the respiratory organs (book lungs or book gills) and the heart.
Unlike arachnids with book lungs (scorpions, most spiders and several others), harvestmen and most other purely tracheate arachnids lack extensive arterial branching and well-defined venous sinuses. The circulatory system consists mainly of a dorsal tubular heart with anterior and posterior aortae. The heart is innervated by a cardiac ganglion.
Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two segments: the cephalothorax (2) and the abdomen (3). The ventral side of a brown widow spider . The epigastric plates and furrow are visible, as well as the hourglass marking on the underside of the abdomen, which is a characteristic feature of widow ...
Basic characteristics of arachnids include four pairs of legs (1) and a body divided into two tagmata: the cephalothorax (2) and the abdomen (3) Almost all adult arachnids have eight legs, unlike adult insects which all have six legs. However, arachnids also have two further pairs of appendages that have become adapted for feeding, defense, and ...
The heart is a tube in the upper part of the body, with a few ostia that act as non-return valves allowing blood to enter the heart from the hemocoel but prevent it from leaving before it reaches the front end. [16]
The mesosoma contains the heart or "dorsal vessel" which is the center of the scorpion's open circulatory system. The heart is continuous with a deep arterial system which spreads throughout the body. Sinuses return deoxygenated blood to the heart; the blood is re-oxygenated by cardiac pores. The mesosoma also contains the reproductive system.
It is the only supra-ordinal group of arachnids that is strongly supported in molecular phylogenetic studies. [1] Two extinct orders are also placed in this clade, Haptopoda and Uraraneida. [2] In 2016, a newly described fossil arachnid, Idmonarachne, was also included in the Tetrapulmonata; as of March 2016 it has not been assigned to an order.
The order Trigonotarbida is a group of extinct arachnids whose fossil record extends from the late Silurian to the early Permian (Pridoli to Sakmarian). [1] [2] [3] These animals are known from several localities in Europe and North America, as well as a single record from Argentina.