<unk> s (Phylum<unk> a, from Greek ἄρθρον arthron, "joint", and ποδός podos, "foot") are the largest phylum of animals and include the insects, arachnids, crustaceans, and others.<unk> s are characterised by the possession of a segmented body with appendages on at least one segment. They have a dorsal heart and a ventral nervous system. All<unk> s are covered by a hard exoskeleton made of chitin, a polysaccharide, which provides physical protection and resistance to desiccation. Periodically, an<unk>  sheds this covering when it molts.
More than 80% of described living animal species are<unk> s, with over a million modern species described and a fossil record reaching back to the late proterozoic era.<unk> s are common throughout marine, freshwater, terrestrial, and even aerial environments, as well as including various symbiotic and parasitic forms. They range in size from microscopic plankton (~¼ mm) up to forms several metres long. The largest living<unk>  is the Japanese spider crab, with a leg span up to 3½ m (12 ft), and some prehistoric<unk> s were even larger, such as Jaekelopterus and Arthropleura.
Arthropod
Vertebrate
Flea
Slug
Parasitic worm
Hookworm
Oily fish
Cestoda