Arthropods and Echinoderms

Biology I: Chapter 28

ARTHROPODS

Arthropods

• Phylum Arthropoda

• Insects, crabs, centipedes, spiders

• Segmented body

• Tough exoskeleton made of chitin

• Jointed appendages

Evolution of Arthropods

• Fewer body segments

• Highly specialized appendages for feeding, movement and other functions

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

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

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

Respiration: Aquatic Arthropods

• Respire through feather-like gills (i.e. lobster and crabs)

• The horseshoe crab respires through organs: book gills

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

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

Response

• Well-developed nervous system

• Brain

• Sophisticated sense organs, (i.e. eyes and taste receptors)

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

Reproduction

Terrestrial Arthropods

• Internal fertilization

• Sperm or sperm packet

Aquatic Arthropods

• Internal or external fertilization

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

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

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

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

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

Spiders and Their Relatives

• Chelicerates

• Mouthparts: chelicerae

• 2 body sections

• 4 pairs of walking legs

• Horseshoe crabs, spiders, ticks, and scorpions

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

ECHINODERMS

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

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

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

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

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

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

Movement

• Most move by tube feet and thin layers of muscle fibers attached to their endoskeleton

• Mobility is determined by the kind of endoskeleton

Reproduction

• Reproduce by external fertilization

• The sexes are separate in most sea star species

Groups of Echinoderms

• 7000 species

– Sea urchins and sand dollars

– Brittle stars

– Sea cucumbers

– Sea stars

– Sea lilies and feather stars

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