What phylum do arachnids belong to?

Arachnids belong to the phylum Arthropoda. Spiders, scorpions, ticks, mites, harvestmen, and several related groups are all arachnids, but they are only one branch of the enormous arthropod phylum.

Arthropods are defined by features including a segmented body, paired jointed appendages, and an external skeleton, or exoskeleton, largely made from chitin. Because the exoskeleton cannot stretch continuously as the animal grows, arthropods must moult: they shed the old covering and expand before the new one hardens.

Arachnids are classified more narrowly in the class Arachnida and the subphylum Chelicerata. Spiders differ from insects in several familiar ways: adult spiders generally have eight legs rather than six, lack antennae, and have bodies organized into two main tagmata rather than the three main regions typical of insects. Other arthropods include crustaceans, insects, centipedes, millipedes, and extinct trilobites. Arthropoda is regarded as the largest animal phylum, with an extraordinary range of body plans and habitats.