Chapter 40: Echinoderms and Invertebrate Chordates

40-1 Phylum Echinodermata

40-2 Invertebrate Chordates

I. Characteristics of Echinoderms (~7,000 marine species, 500 mya)• Radial, endoskeleton, NO cephalization, deuterostomes, separate sexes, regenerative, few sessile, and ALL develop as bilateral LARVAE.

40-1 Phylum Echinodermata

(1) Pentaradial Symmetry

• Body parts EXTEND from center along FIVE rays.

(2) Ossicles (make up ENDOSKELETON)

• Rigid PLATES of CaCO3 attached to SPINES that protrude through skin (i.e., spiny skin)

(1) Scientists have found many echinoderm fossils from the Cambrian period, but they have found few fossils of other species that must have lived during the Cambrian period. What might explain the large number of fossilized echinoderms?

Critical Thinking

(3) Water-Vascular System• Water-filled CANALS designed for LOCOMOTION and TRANSPORT.

(4) Tube Feet• EXTENSIONS of WVS USED for movement, feeding, respiration, and excretion.

II. Classification of Phylum Echinodermata• Classified into FIVE CLASSES on MORPHOLOGY and DEVELOPMENT.

                                 

                           

                                     

sea cucumber starfish sea urchin

(A) Class Crinoidea (e.g., sea lily, feather star)• Arms EXTEND from body and branch OUT; MOUTH faces UP

(NOTE: In most echinoderms mouth faces DOWN, toward sea BOTTOM.

(2) Sea cucumbers and sea lilies are relatively sessile animals. Their larvae, however, are capable of swimming. What advantage might swimming larvae provide for these echinoderms?

Critical Thinking

(B) Class Ophiuroidea (e.g., basket stars, brittle stars “Snake-Like”)• LONG, narrow arms QUICK movement along sea floor RAKE IN food with tube feet.

(3) Great populations of plankton rise to the surface of the ocean at night and return to the ocean depths during the day. Basket stars are also active at night, when they uncoil their think, flexible arms. During the day, basket stars curl up and become a compact mass. Why do you think basket stars uncoil their arms at night and coil up during the day?

Critical Thinking

(C) Class Echinoidea (e.g., sea urchins, sand dollars, “Spine-Like”)• Barbed spines feed by SCRAPING algae; Sand dollars have a FLAT, round shape BURROWING in shallow sand.

(1) Test• Internal organs are PROTECTED within a rigid endoskeleton

(i.e., test = skeleton).

(4) Sea urchin eggs (roe) are highly prized in Japan for sushi. In trying to supply the Japanese market for this delicacy, divers have nearly wiped out sea urchin populations in some areas of California. What steps do you suppose could be taken to reestablish sea urchin populations in those areas?

Critical Thinking

(D) Class Holothuroidea (e.g., sea cucumbers, “Water-Polyp”)• ARMLESS, on sea BOTTOM, burrow into soft sediment; TENTACLES sweep food into MOUTH.

(5) Commercial oyster farmers used to remove sea stars from their oyster beds, chop them in half, and throw them back into the water. Would this method be successful in preventing the sea star’ predatory ways against the oyster? Explain your reasoning.

Critical Thinking

(E) Class Asteroidea (e.g., sea stars, starfish, “Star-like”)• ROCKY coastlines, varied COLORS, up to 24 arms and PREY upon BIVALVES (clams, oysters, mussels, etc…).

III. Structure and Function of Echinoderms• Sea stars have STRUCTURAL features UNIQUE to echinodermata.

(A) External Structure• Flattened body, TWO rows of TUBE FEET under EACH arm.

(1) Oral & Aboral Surfaces• Oral is UNDERSIDE; aboral is TOP of body.

(2) Pedicellariae (i.e., tiny pincers)

• Keeps BODY SURFACE FREE of foreign material (e.g., algae, barnacles)

(B) Water-Vascular System• Network of water-filled CANALS that CONNECT to TUBE FEET.

(1) Madreporite• ABORAL surface contains PORES where water ENTERS vascular system.

(2) Stone Canal, Ring Canal, & Radial Canal• Water moves into madreporite into EACH arm and extends TUBE FEET.

(3) Ampulla (i.e., BULB-like sac at END of each TUBE FOOT)

• Forces water into tube feet and allows for extension AND suction.

(C) Feeding and Digestion• PREDATORS use SUCTION of tube feet to BOTH halves of shell to PULL OPEN.

(1) Cardiac Stomach (mouth-esophagus-cardiac stomach)• Turns INSIDE-OUT into bivalve to DIGEST soft tissues INSIDE shell.

(2) Pyloric Stomach (receives partially digested food from cardiac stomach)• COMPLETES food digestion inside body of sea star.

(D) Other Body Systems• FLUID in coelom bathes organs AND spreads nutrients and OXYGEN.

(1) Skin Gills (hollow tubes that project from the coelom lining)• GAS EXCHANGE and WASTE REMOVAL through thin walls of skin gills AND tube feet (i.e., DIFFUSION).

(2) Nerve Ring (mouth) & Radial Nerve (arms) (a primitive system)• NERVE NET controls movements of spines, pedicellariae, and skin gills.

(NOTE: Each arm also has an EYESPOT used to detect light)

(E) Reproduction and Development (SEPARATE sexes)• ARMS of sea stars make SPERM or EGGS, EXTERNAL fertilization in WATER. (Each ZYGOTE results in a“bipinnaria”).

(1) Bipinnaria• BILATERAL free-swimming larva settles to BOTTOM (after 2 months) and DEVELOPS into a RADIAL adult.

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I. Characteristics (marine invertebrates are ONLY 5% of ALL chordates)(1) Notochord (develops into vertebral column in higher chordates)

40-2 Invertebrate Chordates

(2) Dorsal nerve cord

(3) Pharyngeal pouches (develop into gills-aquatic OR lungs-terrestrial)(4) Postanal tail

II. Evolution and Classification (THREE subphyla)• Deuterostomes (like echinoderms); exclusively marine (invertebrate chordates).

(A) Subphylum Cephalochordata (~24 species of marine lancelets)• Inhabit warm, shallow saltwater using muscular tail to wriggle backward in sand; CILIA draw food-laden water into pharynx for DIGESTION.

(1) Atriapore• Water FILTERED for food particles is EXPELLED through this opening LEAVING the lancelet body.

(B) Subphylum Urochordata (~2,000 species of Tunicates—Sea Squirts)• Hermaphroditic, sessile, barrel-shaped adapted for filter-feeding LOST ALL but ONE characteristic during LARVAL development. (i.e., ONLY pharyngeal pouch-like slits are RETAINED in adult)

Extra Slides AND Answers for Critical Thinking Questions

(1) Swimming larvae allow these echinoderms to disperse widely. Thus, they may be able to spread out to other areas where there may be more food or fewer predators.

(2) No this was not a good method. A new sea star may grow from an arm if part of the central region is still attached. A better method might be to raise the oysters in cages covered with wire mesh, which would keep the sea stars out but allow smaller organisms that the oysters feed on to enter.

(3) With their hard endoskeletons, echinoderms would have been more easily fossilized than soft-bodied animals.

(4) Harvesting seasons could be set up, with limits on the number and size of sea urchins that could be collected. These limits would allow the animals to reproduce and increase their populations.

(5) Basket stars are filter feeders, so they have much more food available to them at night. By curling up during the day, they make themselves less noticeable to predators.