Phylum ArthropodaIntroduction

• 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

ArthropodaGeneral Characteristics

• When compared to annelids– both are metameric– primitively have single pair of

appendages/segment– nervous system along the same plan– similar embryonic development

ArthropodaGeneral Characteristics

• metameric body plan– divided into tagmata (i.e.,

tagmatization)

– somites fused into functional units• head

• thorax

• abdomen

– or• cephalothorax

• abdomen

ArthropodaGeneral Characteristics

Appendageslots 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