phylum+arthropoda+1

PHYLUM ARTHROPODA

FACTS:
 * Name means 'jointed foot'
 * More than 3/4 of all known species of animals are arthropods--900,000
 * Japanese crab //Macrocheira// is the largest arthropod at 4 meters across
 * A parasitic mite //Demodex// is the smallest arthropod at 0.1 mm long
 * Found in all types of environments,some species live in places where no other animal could survive

SPECIAL CHARACTERISTICS: 1) Exoskeleton 2) Specialized Metameres and Appendages 3) Tracheae or gills 4) Metamorphosis
 * Arthropods have an outer skeleton (cuticle) containing chitin, a protein/polysaccharide compound
 * Most crustaceans (lobsters, crabs, shrimp, crayfish) have a cuticle that also contains calcium salts for hardness
 * Must be shed for growth--This process is called ecdysis, or molting
 * Segments (somites or metameres) are often fused for functionality
 * Typically one pair of appendages per somite, but the number is often modified
 * Appendages are often adapted for sensory functions, food handling, walking, and swimming
 * In terrestrial species, air tubes called tracheae deliver oxygen directly to the cells
 * Aquatic arthropods usually breathe with gills
 * Many arthropods go through several body changes in which the larva is very different from the adult
 * These differences reduce competition for food and space

GENERAL CHARACTERISITICS
 * Bilateral symmetry
 * Coelom is present, but reduced; the body cavity consists of a hemocoel, or a blood-filled space
 * Open circulatory system; blood flows through a heart, arteries and into the hemocoel
 * Digestive, excretory, and nervous systems well-developed
 * Sexes usually separate (dioecious) with internal fertilization
 * Most are herbivores

ARTHROPOD CLASSIFICATION

SUBPHYLUM TRILOBITA
 * Trilobites
 * All extinct

SUBPHYLUM CHELICERATA--six pairs of appendages CLASS MEROSTOMATA--Horseshoe crabs CLASS ARACHNIDA--Scorpions, ticks, mites, spiders
 * Most are predators
 * Most have poison glands or stingers
 * Most are harmless to humans and help get rid of insects; however, some carry diseases or cause painful bites

EXAMPLES OF ARACHNIDS 1) Spiders
 * 35,000 species
 * All are predators and usually eat insects
 * Breathe with book lungs or tracheae
 * Usually have 8 simple eyes, used for detecting movement
 * Sensory setae, or hairs are used to gather information about the surroundings
 * Abdominal silk glands and spinnerets produce silk thread that is stronger that steel threads of the same diameter
 * Not all spiders spin webs--wolf spiders, jumping spiders, and fisher spiders don't
 * The only spiders in the US that can give fatal bites are the black widow, //Latrodectus mactans,// and the brown recluse, //Loxosceles reclusa
 * //Black widows, which have an orange or red hourglass on the underside of the abdomen, have a neurotoxic venom which kills 4-5 people per 1000 bites
 * Brown recluse spiders are brown with a violin-shaped dorsal stripe and produce a hemolytic toxin that kills tissues and is rarely fatal

2) Scorpions
 * Usually hide during the day & feed at night
 * Feed mainly on insects & spiders
 * A stinging apparatus is present at the end of the postabdomen, or tail
 * Most are not harmful to humans, some species in Mexico and Africa can be fatal

3) Harvestmen (Daddy Longlegs)
 * Have 4 pairs of legs
 * Often are scavengers

4) Ticks and Mites
 * Most medically and economically important group of arachnids
 * About 30,000 species described; an estimated 500,000--1 million exist
 * Many are parasitic
 * No external segments
 * Usually only 4 pairs of legs in adults
 * Include dust mites, chiggers (redbugs), hair follicle mites (most of us have them), ticks which cause Lyme disease & Rocky Mountain spotted fever

SUBPHYLUM CRUSTACEA CLASS BRANCHIOPODA--Water fleas, brine shrimp, fairy shrimp CLASS MAXILLOPODA--Copepods and barnacles CLASS MALACOSTRACA--Roly polys, crayfish, crabs, lobsters, 'true' shrimp
 * mainly freshwater; most abundant organisms in freshwater zooplankton
 * Copepods are the most abundant organisms in marine zooplankton
 * Crayfish, crabs, lobsters, and shrimp belong to the Order Decapoda; they usually have large pincers called chelae or chelipeds

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