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Phylum Arthropoda It doesn’t get any bigger than this! Major Features • An exoskeleton (external skeleton) made out of chitin. • Must molt (shed their skeleton) in order to grow. • Have jointed appendages – Serve a variety of roles (walking, swimming, repro, eating, sensing) • Three body regions – head, thorax, abdomen. Features cont’d • Well-developed nervous system – Includes a brain and a ventral nerve cord – Includes sense organs (antennae, compound eyes) Diversity Classification • Broken into subphyla. – Subphylum Crustacea – Subphylum Uniramia – Subphylum Chelicerata Subphylum Crustacea • “Crustaceans” – barnacles, shrimp, lobster, crab, crayfish… marine arthropods. • The sow bug is a terrestrial example. Crustacea cont’d (We’ll use crayfish as our representative) External structure: • Cephalothorax – skeleton over head and thorax are fused. • Head has compound eyes, antennae, and pairs of mouth appendages. • Thorax has 5 pairs of limbs (4 pairs of walking legs, 1 pair of pincers [chelipeds]). • Abdomen has swimmerets, uropods and telson. Crustacea cont’d Crustacea cont’d Internal structure: • Digestive system – 2-chambered stomach, digestive glands, intestine. Green glands for excretion. • Cardiovascular system – heart pumps blood into the space surrounding the internal organs (the “hemocoel”). – Blood contains a blue pigment (hemocyanin). Crustacean circulation Crustacea cont’d Internal structure cont’d: • Nervous system – brain, ventral nerve cord, and ganglia in several segments. – Receptors line antennae – include chemical receptors and force receptors. • Reproduction – separate genders. – Sperm transferred by 1st pair of swimmerets. – Females carry fertilized eggs on their swimmerets. Crustacea cont’d Subphylum Uniramia • “Insects” – most diverse group on Earth. General structure: • Head has compound or simple eyes, antennae, and mouth appendages. • Thorax has 3 pairs of legs – Sometimes includes wings. • Abdomen stores the internal organs. Insects cont’d We will use grasshoppers as our representative. External structure: • 3rd pair of legs adapted for jumping. • Has two pairs of wings. • Females – have a posterior structure called an ovipositor for digging holes to lay eggs in. • Has tympanum – a thin membrane – on abdomen for hearing. Grasshopper Exterior Insects cont’d Internal Structure: • Digestive system – complete, with mouth, stomach, intestine, and anus. • Excretion – uses structures called Malpighian tubules that release uric acid into intestines for disposal. • Respiration – uses openings in the exoskeleton called spiracles that that lead into a trachea. – Air is pumped by the contraction and relaxation of the body wall. Spiracles and Malpighian Tubules Insects cont’d • Circulation – a heart pumps hemolymph into the aorta, which empties into a hemocoel (open space around organs). – Hemolymph in insects is NOT used to carry O2, so it has no pigment. • Reproduction – Fertilization is internal. Genders are separate. – Fertilized eggs are ejected into the ground. Circulation and Reproduction Insects cont’d • Metamorphosis: a change in physiology and anatomy that occurs as an insect matures from a larva to an adult. – (Many insect larvae, including grasshoppers, are called nymphs). Subphylum Chelicerata Arachnids (spiders, ticks, scorpions, mites). General Features: • Cephalothorax – Has 6 pairs of appendages attached to it • 4 pairs are walking legs • 1 pair are pedipalps – these sense and hold prey • 1 pair are fangs (chelicerae) • Abdomen – stores internal organs Arachnid structure More arachnids! Arachnids cont’d (Representative organism = spiders) • Capable of delivering venom to prey through fangs. • Digestion: prey is injected with digestive juices. Juices gradually digest prey. – Spider will “suck up” liquefied prey to complete digestion. Arachnids cont’d • Respiration: Uses “book lungs” – folds of tissue inside the body wall. – The folded surface provides plenty of room for gas exchange.