Phylum Arthropoda – Phylum and Subphyla Characteristics Phylum Characteristics: segmented body (tagmata), chitinous exoskeleton; jointed appendages; highly developed sense organs and complex behavior patterns (incl. social systems) Overview of Diversity and Characteristics of Subphyla Subphylum Trilobita: extinct trilobites (most common in the Paleozoic); cephalon (head segment) with compound eyes, mouth, and antennae Subphylum Chelicerata: extinct eurypterids (up to 3 m long), horseshoe crabs, sea spiders, spiders, scorpions, ticks, and mites; six pairs of cephalothoracic appendages (incl. pair of chelicerae for sucking liquid food), plus four pairs of walking legs; lack antennae Subphylum Myriapoda: centipedes, millipedes, others; mandibles; one or two pairs of uniramous appendages on most or all trunk segments Subphylum Crustacea: crabs, shrimps, and other aquatic forms; biramous appendages, often modified; gills present; carapace Subphylum Hexapoda: insects; head, thorax, and abdomen; three pairs of legs; wings often present; highly modified mouthparts

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Subphyla Chelicerata and Myriapoda Subphylum Chelicerata (extant forms) Horseshoe crabs: hinged carapace and tail-like telson; book gills; “trilobite” larvae; mass spawnings provide food for shorebirds Sea spiders (pycnogonids): most abundant in polar oceans; some duplicated segments result in 5-6 pairs of legs Class Arachnida (earliest terrestrial animals, ~ Silurian) Spiders: cephalothorax and abdomen joined by slender pedicel; fangs with venom glands (incl. black widow, brown recluse, tarantulas); many with silk glands and webs; eight simple eyes; control populations of insects Scorpions: stingers with venom glands (some can be fatal); complex mating dance; viviparous development, young develop on female’s back Ticks and mites: fusion of tagmata; ticks suck blood and can spread disease (ex. Lyme disease); mites important agricultural pests; include commensal hair follicle mite (Demodex) and itch mite (cause of scabies) Sun and whip scorpions: include local “camel spiders” Subphylum Myriapoda: paired appendages on nearly all trunk segments Class Chilopoda: centipedes; venomous, active carnivores (tropical species can be dangerous to humans); up to 177 segments with most bearing pair of jointed legs Class Diplopoda: millipedes; slow, herbivorous; secrete toxic or repellent fluids from body; abdominal segments with two pairs of legs (the appearance of a thousand feet)

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Fig Sonograph of Sea Spider Fossil

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Fig and 19.19

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The Crustaceans: Diversity and Class Characteristics Diversity: “insects of the sea”; about 67,000 described species, some extremely abundant (ex. copepods) Class Remipedia: inhabit caves with connections to the sea Class Branchiopoda: include brine shrimp and water fleas (Daphnia) Class Ostracoda (ostracods): bivalved carapace; many bioluminescent Class Maxillopoda: include copepods (planktonic and parasitic forms), fish lice, and barnacles (sessile or parasitic as adults) Class Malacostraca: most with hard carapace covering cephalothorax Order Isopoda: dorsoventrally flattened, lack carapace; include “pill bugs”, rock lice, parasitic forms, and deep-sea forms (some of which exhibit gigantism) Order Amphipoda: laterally flattened; lack carapace; include “kelp hoppers”, skeleton shrimp, whale lice, and pelagic species (ex. Phronima) Order Euphausiacea (krills): carapace fused to thoracic segments but gills partially exposed; abundant prey in Southern Ocean for baleen whales, penguins, crabeater seals, and others Order Decapoda: crabs, shrimp, lobsters, crayfish; often with chelae (pincers); support important fisheries (ex. Alaskan king crab)

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The Crustaceans: Structure and Development Structure: two pairs of antennae, biramous appendages Serial homology and specialization of appendages (incl. mouthparts); decapods with five pairs of walking legs, abdominal swimmerets, and tail-like uropod Gills in larger forms extend from appendages or are internal (water pumped into gill cavities via pumping of second maxillae) Open circulatory system; hemoglobin or hemocyanin (copper-based) Sensory systems include statocysts, tactile hairs, and compound eyes Development: metamorphosis vs. direct development Common larval stages include nauplius (with single medial eye), zoea (crabs), cyprid (barnacles), and phyllasoma (lobsters) Molting cycle and ecdysis (shedding of cuticle) controlled by hormones Cuticle is non-living, must be shed as grows; growth occurs during intermolt phases until space filled, absorbed water splits old cuticle, animal stays sheltered until new cuticle hardens

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