Echinoderms and Invertebrate Chordates

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Echinoderms and Invertebrate Chordates
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Phylum Echinodermata

• Name means spiny skin
• Include organisms such as
• Asteroidea sea stars
• Echinoidea - sea urchins, sand dollars
• Crinoidea - sea lilies
• Ophiuroidea - brittle stars
• Holothuroidea - sea cucumbers.
• All inhabit marine environments

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Characteristics

• Five-part radial symmetry as adults, bilateral
  symmetry as larvae
• Radial symmetry helps slow or fixed animals sense
  food or predators from all directions
• Endoskeleton
• For support and protection
• Water-vascular system
• Movement, exchange of gases, capture food, and
  excrete wastes
• Coelomic circulation and respiration

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Endoskeleton

• calcium carbonate (calcite) plates, ossicles, and
  spines that are covered with a thin layer of skin
• Some spines on sea stars are modified to into
  pincer like appendages- pedicellaria, that
  protect and clean the surface of a sea star
• They can contract muscles to tighten their skin
  and thereby make the calcite plates rigid, or
  relax the muscles to enable flexibility

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Tube Feet

• Water filled connected canals of thin walled
  tubes that end in a suction cup tube feet. Each
  works independently the animal moves by pushing
  out, then pulling in its tube feet.
• A hydraulic system under water pressure. Water
  enters through a madreporite plate
• Carry out gas exchange and excretion by diffusion
  through the tube feet

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Coelomic respiration

• Ceolomic cavity serves both repiratory and
  circulatory functions
• Gases move through fluid filled coelom
• Skin gills grow between the spines increasing
  surface area for gas exchange
• Skin gills function as excretory structures,
  wastes are excreted

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Varied methods of nutrition

• Sea stars are carnivorous, prey on clams
• The suction caused by contraction and relaxation
  of the ampulae is very strong, they can open a
  clam shell
• Sea urchins are herbivores, graze on algae
• Brittle stars, sea lilies, sea cucumbers feed on
  dead and decaying matter

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Sea star stomach

• To eat a sea star pushes its stomach out of its
  mouth and spreads it over the food. Enzymes
  secreted by a digestive gland turn solid food
  into liquid that the stomach absorbs, then it
  pulls its stomach back in.
• Wastes of digestion are expelled through the anus

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Nervous system

• No head or brain
• Central nerve ring that extends to form a nerve
  net down each ray of a sea star.
• Cells that detect touch and light, an eyespot,
  exists at the end of a sea stars ray, they turn
  up the end of the ray to detect light
• Most do not have sensory organs

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Evolutionary relationships

• Echinoderms are closely related to chordatesThey
  share a common ancestor
• The larvae have bilateral symmetry, an indication
  that their ancestors may have been bilateral.
• Deuterostomes development in echinoderms As in
  chordates, the anus develops at or near the
  blastopore of the grastrula.
• endoskeleton

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Echinoderm development
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Invertebrate Chordate

• Phylum Chordata
• Three subphyla
• Urochordata the tunicates
• Chephalochordata the lancelets
• Vertebrata the vertebrates
• Chordate criteria
• Notochord
• Dorsal hollow nerve cord
• Gill slits
• Post anal tail
• Muscle blocks

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Notochord

• Rodlike structure located between digestive
  system and dorsal hollow nerve cord.
• Retained until adulthood in invertebrate
  chordates
• Replaced by backbone in vertebrates
• Develops from mesoderm
• Provides support so that side to side movement
  can propel the animal forward

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Dorsal hollow nerve cord

• Develops from ectoderm that forms a tube composed
  of cells surrounding a fluid filled canal that
  lies above the notochord. These cells develop
  into the spinal cord.
• Cells in anterior portion of cord develop into
  brain
• Nerves connect the nerve cord to each block of
  muscles.

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Gill Slits

• Paired openings located in the pharynx behind
  mouth.
• Many chordates only have gill slits during
  embryonic development
• Those that have gill slits as adults, strain food
  from water the fishes have internal gills that
  exchange gases during respiration

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Muscle blocks

• Modified body segments that are formed from
  stacked muscle layers
• In cooked fish these are easily separated
• Anchored by notochord they can pull with enough
  tension to be firm
• Aid in movement of a muscular tail
• In humans that tail disappears as the embryo
  develops
• In most non chordates with tails the digestive
  system exists to the tip of the tail
• In chordates the tail extend past the anus

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Diversity - Tunicates

• Subphylum Urochordata - Tunicates or sea squirts
• Sessile, filter feeding Larval stage appears more
  like chordates
• Gill slits in adults
• Free swimming larvae, sessile adult
• Secrete a tough sac of cellulose looks like a
  tunic
• When disturbed will squirt out water
• Hermaphrodites, budding

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Lancelets

• Subphylum Cephalochordata.
• Small, fish-like, no scales, repeating segments,
  live in sand with heads sticking out
• Retain all chordate features throughout life
• Filter feeders
• Separate sexes

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Origin of chordates

• Fossils resembling lancelets date back to 550
  mya.
• Echinoderms and invertebrate chordates arose from
  common ancestor
• Other characteristics shared with some non
  chordates
• Bilateral symmetry
• segmentation

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Echinodermata and Chordata

• Close evolutionary relationship between
  echinoderms and vertebrates.
• Similar embryologic development.
• Similar biochemical activity associated with
  muscle activity and oxygen-carrying blood
  pigments.

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Early Remains of Chordata

• Cathaymyrus L. found in the 10 million year older
  (535 m.y.) Chengjiang fossil site in China and
  Pikaia, M. Cambrian may lie at the start of the
  evolution of the fishes.

Pikaia