Phylum Arthropoda – Phylum and Subphyla Characteristics

Phylum Arthropoda – Phylum and Subphyla Characteristics • Phylum Characteristics: segmented body (tagmata), chitinousexoskeleton; 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

Fig. 19.4

Fig. 19.1

Fig. 19.2

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. 19.7 Sonograph of Sea Spider Fossil

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Fig. 19.18 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)

Fig. 20.1

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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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Phylum Arthropoda – Phylum and Subphyla Characteristics