(Latin: a suffix used to form names of zoological groups, classes, and orders)

The oldest known reptiles: turtles; extinct Permian forms.
Arachnida (proper noun)
A class of arthropods of the subphylum Chelicerata, that includes mites, ticks, spiders, scorpions, and related forms: Many of the most important parasites and vectors of human and animal pathogens are included in this large assemblage termed Arachnida.

More about Arthropoda

Along with the insects, crustaceans, centipedes, and millipedes; spiders are members of that group of animals without backbones referred to as the Arthropoda, literally the "jointed-limbed" animals.

Clearly they lack a backbone and instead have an external skeleton, called an "exoskeleton", which has some similarities to a suit of armor; it is tough and fairly rigid and the muscles are attached to it internally.

Like the vertebrate skeleton, that of the arthopods is designed as a compromise between rigidity, to provide support and protection for the soft, delicate internal organs, and flexibility, to allow for ease of movement.

Spiders of the World by Rod & Ken Preston-Mafham;
Facts On File Publication; New York; 1984; page 16.
Araneida (proper noun)
An order of the class Arachnida: All the modern spiders, except for the Uraraneida, are included in the order Araneida.
A grand division of the Vertebrata, including the Amphibia and Fishes.
A group of extinct Reptilia, intermediate in structure (especially with regard to the pelvis) between reptiles and birds.
Pseudoscorpions are tiny scorpion-like arthropods, with a flat, pear-shaped body with two sections, eight five-segmented legs, and simple eyes.

The color of the body can be yellowish-tan to dark-brown, with the paired claws often a contrasting color. They have two very long pedipalps, or pincers, which strongly resemble the scorpion's claws, but the pseudoscorpion's abdomen is short and rounded at the rear, rather than extending into a segmented tail and sting.

The movable part of the pincer contains a venom gland and duct; the poison is used to capture and immobilize their tiny prey. They do not bite.

To digest prey, they pour a mildly corrosive fluid over the prey, then ingest the liquefied remains.

They spin silk from a gland in their jaws to make disk-shaped cocoons for mating, molting, or enduring cold weather.

A comprehensive group of vertebrates, including the reptiles and birds.
The scorpions; an order of venomous, predaceous, arachnid arthropods characterised by a distinctly segmented bony abdomen terminating in a sharply recurved stinging spine equipped with a poison gland; causes a severely painful but rarely fatal sting. North American genera include Centruroides, Hadrurus, and Vejovis.
The extinct mammal-like reptiles found inhabiting all continents from the mid Permian to late Triassic.