The Mesothelae are a suborder of spider

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

s (Order Araneae) that includes the extinct families Arthrolycosidae

Arthrolycosidae

Arthrolycosidae is an extinct family of primitive spiders that bear some resemblance to the wolf spiders....

 and Arthromygalidae

Arthromygalidae

The Arthromygalidae are an extinct family of primitive spiders that bear some resemblance to the tarantulas, at its peak in the Carboniferous, some 280 million years ago....

 and the only extant family Liphistiidae

Liphistiidae

The spider family Liphistiidae comprises 5 genera and 85 species from Southeast Asia, China, and Japan. They are among the most basal living spiders, belonging to the suborder Mesothelae...

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The Mesothelae is thought to form the sister group to all other living spiders, representing an ancient and "primitive" line of spiders. Recent Mesothelae are characterized by the narrow sternum

Sternum

The sternum or breastbone is a long flat bony plate shaped like a capital "T" located anteriorly to the heart in the center of the thorax...

 on the ventral side of the prosoma. Several plesiomorphic characteristics may be useful in recognizing these spiders: there are tergite plates on the dorsal

Dorsum (biology)

In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...

 side and the almost-median

Median

In probability theory and statistics, a median is described as the numerical value separating the higher half of a sample, a population, or a probability distribution, from the lower half. The median of a finite list of numbers can be found by arranging all the observations from lowest value to...

 position of the spinnerets on the ventral side of the opisthosoma

Opisthosoma

The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...

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