Phylum Arthropoda

Phylum Arthropoda

Arthropoda is the largest and most diverse animal phylum, defined by a hard, chitinous exoskeleton (which must be periodically moulted to allow growth), jointed appendages (the phylum's name literally means "jointed feet"), and an open circulatory system in which blood does not flow entirely within closed vessels but instead bathes the internal organs directly within large, blood-filled body-cavity spaces called the haemocoel — the arthropod body cavity, distinct from the true, mesoderm-lined coelom of annelids, since much of the arthropod body cavity is derived from the embryonic blood sinuses (haemocoelic in origin) rather than being a true coelom.

The major arthropod groups covered here include:

• Class Crustacea — mostly aquatic arthropods (crabs, prawns, lobsters) typically bearing two pairs of sensory antennae as their first two pairs of appendages (a first, shorter pair — the antennules — and a second, longer pair — the antennae proper), a feature that distinguishes them from insects and most other arthropod groups, which bear only one pair of antennae (or none).
• Subphylum Chelicerata — arthropods lacking antennae altogether and bearing a pair of pincer- or fang-like appendages (chelicerae) instead, as their most anterior appendages. This subphylum includes the scorpion, whose cephalic (head-region) appendages consist of a pair of chelicerae (small pincers near the mouth) and a pair of large, prominent pedipalps (the scorpion's conspicuous grasping pincers); the horseshoe crab, Limulus, often called a "living fossil" because its body form has remained almost unchanged for several hundred million years, since its fossil relatives are known from very ancient rock strata while its living descendants show barely any structural change; and the ticks and mites, small chelicerate arachnids of considerable medical and veterinary importance as parasites and disease vectors. Both the scorpion and Limulus possess distinctive respiratory structures: the scorpion breathes using book lungs, thin, stacked leaf-like membranes housed in an internal chamber that resemble the pages of a book, while Limulus breathes using book gills, an externally exposed, structurally similar stack of thin leaf-like respiratory plates adapted for use in water rather than air.
• Class Insecta — the most species-rich class of arthropods (and of the entire animal kingdom), typically bearing a single pair of antennae, three pairs of walking legs, and (in most groups) wings. …