The arrangement of appendages in the largest class, Arachnids, is a pair of chelicerae, a pair of pedipalps, and four pairs of walking legs. The class feeds by pumping digestive enzymes into its prey and sucking up the digested material. What remains is a dried husk of the original prey. The ancient arthropods that are ancestors to horseshoe crabs are trilobites. Horseshoe crabs exhibit external fertilization.
Sea spiders regulate body wastes and gas exchange by direct diffusion across the body surface.
Crustaceans have two pairs of antennae, three pairs of chewing appendages, and various numbers of walking legs. The legs are on the abdomen as well as the thorax. The labyrinth functions to regulate the osmotic condition of the body primarily, and secondarily to excrete salts, amino acids, and water. Millipedes and centipedes are both members of Uniramia, with bodies that consist of a head region followed by numerous segments with paired appendages. However, centipedes have one pair of legs per segment and are carnivores.