The main distinguishing feature of a pseudarthropoid is its mesoskeleton. This consists of an exoskeleton with ingrown portions at the margins of each shell segment. The ingrown portions accumulate after each molting cycle, and provide the mesoskeleton with most of its strength. This allows the animal's cuticle to be thin while still providing enough strength to support its weight. The thin cuticle costs relatively little nutrient value to grow, so a pseudarthropoid can molt frequently. Also, because the creature's weight is supported by the permanent ingrown shell portions, it is not vulnerable while molting as a normal insect would be.
Pseudarthropoids have enclosed circulatory systems and centralized respiratory systems, like vertebrates. Were it not for these features, the pressure difference across their height would displace blood and close trachea in their lower extremities. However, the layout of these systems is significantly different than in vertebrates. They typically have multiple hearts located dorsally (where a vertebrate would have its spine), and the respiratory system merges not with the mouth but with the antennae. Since pseudarthropoids retain the vestigial feature of antennae as scent organs, placing their nostrils inside hollow antenna allows them to smell as they inhale.
Like insects, most pseudarthropoids have side-swinging mandibles. However, these mandibles are spanned by flaps of flexible cuticle to hold food in, as large insectoids cannot rely on surface tension as normal-sized insects would.
Pseudarthropoids possess both simple and compound eyes. The simple eyes are basically enlarged versions of compound eye elements, and provide them with the ability to see detail as a vertebrate eye would. The vestigial compound eye elements provide them with superior peripheral vision.
The exact origin of pseudarthropoids is a matter of some debate. While it is known by the traces of magic in their bodies that modern domesticated pseudarthropoids were created by teratogenesis, it is unclear whether the entire pseudarthropoid phylum was created by magic or whether existing pseudarthropoid species were modified to make them more useful or easier to tame. The Locus is after all a large place, and there is a good chance one of its worlds could have evolved such an odd type of organism prior to being merged into the network. On the other hand, the civilization responsible for creating modern pseudarthropoids collapsed a very long time ago leaving minimal written records, so the exact technique employed in their origin is destined to remain a mystery.