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Arthropods and Echinoderms
Biology I: Chapter 28
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ARTHROPODS
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Arthropods
• Phylum Arthropoda
• Insects, crabs, centipedes, spiders
• Segmented body
• Tough exoskeleton made of chitin
• Jointed appendages
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Evolution of Arthropods
• Fewer body segments
• Highly specialized appendages for feeding, movement and other functions
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Form and Function in Arthropods
• Complex organ systems; some only found only in this phylum
• Tracheal tubes (respiration)
• Open circulatory system
• Excrete wastes through saclike tubules
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Feeding
• Herbivores, carnivores, omnivores, bloodsuckers, filter feeders, detritivores, and parasites
• Mouthparts have evolved to allow them to eat anything imaginable
• Pincers or fangs to sickle-shaped jaws that can cut through the tissues of captured prey
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Respiration: Terrestrial Arthropods
• Tracheal tube: one of many branching, air-filled tubes that extend throughout the body
• Spiracle: small opening located along the side of the body through which air enters and leaves the body
• Book lung: organ that has layers of respiratory tissue stacked like the pages of a book: used to exchange gases
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Respiration: Aquatic Arthropods
• Respire through feather-like gills (i.e. lobster and crabs)
• The horseshoe crab respires through organs: book gills
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Circulation
• Open circulatory system
• Well-developed heart pumps blood through arteries that branch and enter the tissues
• Blood leaves the blood vessels and moves through sinuses, or cavities
• The blood recollects in a large sinus surrounding the heart
• It reenters the heart and is again pumped throughout the body
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Excretion
Terrestrial Arthropods
• Malpighian tubules: sac-like organ that extracts wastes from the blood and adds them to feces that move through the gut
Aquatic Arthropods
• Diffusion moves cellular wastes from the arthropod’s body into the surrounding water
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Response
• Well-developed nervous system
• Brain
• Sophisticated sense organs, (i.e. eyes and taste receptors)
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Movement
• Well-developed groups of muscles that are coordinated and controlled by the nervous system
• Allows arthropods to beat their wings against the air to fly, push their legs against the ground to walk, or beat their flippers against the water to swim
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Reproduction
Terrestrial Arthropods
• Internal fertilization
• Sperm or sperm packet
Aquatic Arthropods
• Internal or external fertilization
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Growth and Development in Arthropods
Molting
• As the time for molting approaches, skin glands digest the inner part of the exoskeleton and other glands secrete a new skeleton
• The animal pulls itself out of the remains of the original skeleton…this can take several hours
• The new exoskeleton is soft and the animal is vulnerable to predators
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Groups of Arthropods
• Classified based on the number and structure of their body segments and appendages-particularly their mouthparts
• Crustaceans
• Spiders and their relatives
• Insects and their relatives
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Crustaceans
• 2 pairs of branched antennae
• 2-3 body sections
• Mandibles: chewing mouthparts
• Primarily aquatic
• Examples: Crabs, shrimps, lobsters, crayfishes, and barnacles
• Range in size from small terrestrial pill bugs to spider crabs that have masses around 20 kg
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Crustaceans
• Cephalothorax: region of a crustacean formed by the fusion of the head with the thorax
• Thorax: body part of crustacean that lies just behind the head and houses most of the internal organs
• Abdomen: posterior part of an arthropod’s body
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Crustaceans
• Carapace: the part of the exoskeleton that covers the cephalothorax
• Mandible: mouthpart adapted for biting and grinding food
• Cheliped: one of the first pair of legs of decapods (five pairs of legs: crayfishes, lobsters and crabs)
• Swimmerets: flipper-like appendages used by decapods for swimming
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Spiders and Their Relatives
• Chelicerates
• Mouthparts: chelicerae
• 2 body sections
• 4 pairs of walking legs
• Horseshoe crabs, spiders, ticks, and scorpions
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Horseshoe Crabs
• The oldest living arthropods
• First appeared more than 500 mya and have changed little since that time
• Not true crabs at all! Anatomy is closer to that of spiders
• Chelicerae, five pairs of walking legs, a long spike-like tail used for movement
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Arachnids: Spiders
• The largest group of arachnids
• Spin webs of a strong, flexible protein called silk
• Do not have jaws for chewing: must liquefy food to swallow it
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Arachnids: Spiders
• Chelicerae: pair of mouthparts in chelicerates that contain fangs and are used to stab and paralyze prey
• Pedipalps: pair of mouthparts in chelicerates that are usually modified to grab prey
• Spinneret: organ in spiders that contains silk glands
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Arachnids: Mites and Ticks
• Small arachnids that are usually parasitic
• Chelicerae and pedipalps are specialized for digging into a host’s tissues and sucking out blood or plant fluids
• Mouthparts are so strong that if you try to pull off a tick the cephalothorax may separate from the abdomen and remain in your skin
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Arachnids: Scorpions
• Widespread in warm areas around the world
• Have pedipalps that are enlarged into claws
• The long, segmented abdomen of a scorpion carries a venomous stinger that can kill or paralyze prey
• Chew their prey using their chelicerae
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Insects and Their Relatives
• Uniramians have jaws, one pair of antennae, and unbranched appendages
• A group that contains more species than any other group of animals living today
• Wide variety of forms and lifestyles
– Centipedes– Millipedes
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Insects
• Have a body divided into three parts:– Head– Thorax– Abdomen
• Three pairs of legs are attached to the thorax
• Like all arthropods, insects have a segmented body, exoskeleton, and jointed appendages
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Responses to Stimuli
• Multiple of sense organs are used to respond to stimuli
• Compound eyes
• Chemical receptors for taste and smell on their mouthparts
• Well-developed ears that hear sounds far above the human range
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Adaptations for Feeding
• Three pairs of appendages that are used as mouthparts, including a pair of mandibles
• Adaptations for feeding are not restricted to their mouthparts
• Many produce saliva containing digestive enzymes that help break down food
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Movement and Flight
• 3 pairs of legs
• Used for walking, jumping, flying, capturing and holding prey, etc.
• The evolution of flight has allowed insects to disperse long distances and to colonize a wide variety of habitats
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Metamorphosis
• Incomplete metamorphosis: type of insect development characterized by a similar appearance throughout all stages of the life cycle
• Nymph: immature form that lacks functional sex organs and other adult structures
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Metamorphosis
• Complete metamorphosis: type of insect development in which the larvae look and act nothing like their parents and also feed in completely different ways
• Pupa: stage of metamorphosis in which an insect changes from a larva into an adult
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Insects and Humans
• Many insects are known for their negative effects
• Termites destroy wood structures, moths eat their way through wool clothing, etc.
• Despite their association with destruction and disease, insects contribute to agriculture by pollinating 1/3 of the food that you eat
• Produce commercially valuable silk, wax and honey
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Insect Communication
• Communicate using sound, visual, chemical, and other types of signals
• Much of the communication involves finding a mate
• Pheromones: specific chemical messengers that affect the behavior of development of other individuals of the same species
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Insect Societies
• Ants, bees, termites, and some of their relatives form complex associations called societies
• Society: a group of closely related animals of the same species that work together for the benefit of the whole group
• Caste: group of individual insects specialized to perform particular tasks, or roles
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ECHINODERMS
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Echinoderms
• Phylum Echinodermata
• Live only in the sea
• Spiny skin
• Water vascular system
• Tube feet: suction-cuplike structures
• Endoskeleton: internal skeleton; hardened plates of calcium carbonate
• Five-part radial symmetry
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Form and Function in Echinoderms
• The water vascular system, which is filled with fluid, carries out many essential body functions in echinoderms including respiration, circulation, and movement
• Madreporite: sieve-like structure through which the water vascular system of an echinoderm opens to the outside
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Feeding
• Several methods of feeding
• Sea urchins use five-part jaw-like structures to scrape algae from rocks
• Sea lilies use tube feet to capture floating plankton
• Sea stars feed on mollusks by pushing the stomach out through the mouth
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Respiration and Circulation
• Other than the water vascular system, echinoderms have few adaptations to carry out respiration or circulation
• In most species, the thin-walled tissue of the tube feet provides the main surface for respiration
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Excretion
• Solid wastes are released as feces through the anus
• Nitrogen-containing cellular wastes are excreted primarily in the form of ammonia
• It is passed through the tube feet and skin gills
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Response
• Do not have a highly developed nervous system
• Most have a nerve ring that surrounds the mouth and radial nerves that connect the ring with the body sections
• Most have scattered sensory cells that detect light, gravity, and chemicals released by potential prey
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Movement
• Most move by tube feet and thin layers of muscle fibers attached to their endoskeleton
• Mobility is determined by the kind of endoskeleton
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Reproduction
• Reproduce by external fertilization
• The sexes are separate in most sea star species
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Groups of Echinoderms
• 7000 species
– Sea urchins and sand dollars
– Brittle stars
– Sea cucumbers
– Sea stars
– Sea lilies and feather stars
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Ecology of Echinoderms
• Common in a variety of marine habitats
• A rise or fall of echinoderms can cause major changes to populations of other marine organisms
• Sea urchins control the distribution of algae and other forms of marine life
• Sea stars are important carnivores that control the numbers of other organisms such as clams and corals