This suborder is thought to form the sister group to all other living spiders. Recent Mesothelae are characterized by the narrow sternum on the ventral side of the prosoma. Several plesiomorphic characteristics may be useful in recognizing these spiders: there are tergite plates on the dorsal side and the almost-median position of the spinnerets on the ventral side of the opisthosoma. Although it has been claimed that they lack venom glands and ducts, which almost all other spiders feature, subsequent works have demonstrated that at least some, possibly all, do in fact have both the glands and ducts. All Mesothelae have eight spinnerets in four pairs. Like mygalomorph spiders, they have two pairs of book lungs.
The Heptathelidae were once considered their own family; today they are considered a subfamily of the Liphistiidae (i.e. as Heptathelinae). Unlike all other extant mesothelians, heptathelines do not have fishing lines in front of the entrances to the burrows that they construct, making them more difficult to find. They also have a paired receptaculum (unpaired in other liphistiids), and have a conductor in their palpal organ. These long palps can confusingly look like an extra pair of legs, a mistake also made of some solifugids.
In fact, no spider that large has ever been found: at the time the series began production, the sea scorpionMegarachne had been mistakenly interpreted as a spider. The correct classification was not made until Walking With Monsters was well into production, and the giant spider was left in and called "Mesothelae" instead of Megarachne. Megarachne servinei had up until this time been considered the largest known spider.