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Phylum Arthropoda Introduction Arthropoda General Characteristics • Largest phylum of animals • Over 900,000 species described • includes: – spiders, scorpions, mites, ticks – insects, millipedes, centipedes – crabs, shrimp, lobsters, crayfishes • very adaptive - found in virtually every habitat Arthropoda General Characteristics • When compared to annelids – both are metameric – primitively have single pair of appendages/segment – nervous system along the same plan – similar embryonic development Arthropoda General Characteristics • metameric body plan – divided into tagmata (i.e., tagmatization) – somites fused into functional units • head • thorax • abdomen – or • cephalothorax • abdomen Appendages lots of specialization • locomotion – swimmerets – walking legs • reproduction – gonopods Appendages: primitively with one pair per segment Exoskeleton • made of protein, lipids, chitin, calcium carbonate – advantages: • protection • fights water loss; most successful terrestrial invertebrate group • allows for effective movement on land – disadvantages • heavy and bulky • needs to be periodically replaced (i.e., molting) • vulnerable during molting Selected System Summaries • Complex muscular system that is segmentally arranged • Reduce coelom; mostly a hemocoel filled with blood • Complete digestive tract with mouthparts as modified appendages • Open circulatory system with dorsal contractile heart Respiratory System • Respiration by: – – – – body surfaces - primitive forms gills - crustaceans trachea - insects book lungs - chelicerates Excretion • Most with with paired glands – coxal, antennal or maxillary • insects and others – malpighian tubules Nervous System • similar to that in annelids – dorsal brain (cerebral ganglia) around esophagus – double ventral nerve cords – well developed sensory organs Reproduction • • • • • usually dioecious usually internal fertilization oviparous or ovoviviparous often with metamorphosis some have parthenogenesis WHY ARTHROPODS ARE SO SUCCESSFUL? • versatile exoskeleton • segmentation and appendages for efficient locomotion • air piped to cells directly (insects) • highly developed sensory organs • complex behavior patterns • reduced competition through metamorphosis
