Chaetognaths, Echinoderms, and Hemichordates

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Chaetognaths, Echinoderms, and Hemichordates

• Chapter 22

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Deuterostomes

• Deuterostome characteristics
• Radial, indeterminate cleavage
• Formation of the mouth from a second opening
• Enterocoelous coelom development
• Chaetognaths are placed outside both protostome
  deuterostome groups.

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Phylum Chaetognatha

• The arrow worms, phylum Chaetognatha, are all
  marine, planktonic organisms.
• Some deuterostome embryological characters.
• Molecular works suggests they are protostomes.
• Currently not considered to be part of either
  group.

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Clade Ambulacraria

• Superphylum Ambulacraria contains two
  deuterostome phyla
• Echinodermata and Hemichordata
• Members share a three-part (tripartite) coelom,
  similar larval forms, and an axial complex
  (specialized metaneprhidium).
• Xenoturbella is the sister taxon to Ambulacraria.
• Now considered to be a Phylum Xenoturbellida

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Phylum Echinodermata

• Echinoderms include sea stars, brittle stars, sea
  urchins, crinoids, sea cucumbers.
• Entirely marine
• Lack ability to osmoregulate.
• Almost entirely benthic.
• Nonsegmented.

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Phylum Echinodermata

• Five extant classes of echinoderms are currently
  recognized.

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Phylum Echinodermata Characteristics

• Spiny endoskeleton of plates
• Water vascular system
• Pedicellariae
• Dermal branchiae (skin gills)
• Pentaradial symmetry in adults

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Phylum Echinodermata - Symmetry

• Echinoderms are bilaterally symmetrical as
  larvae.
• This means their ancestors were bilaterally
  symmetrical.

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Phylum Echinodermata - Symmetry

• As adults they show secondary radial symmetry
  pentaradial (5 parts).
• Perhaps an adaptation for sessile living in early
  echinoderms.
• Crinoids

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Phylum Echinodermata - Symmetry

• Todays echinoderms are mostly motile.
• Many are still radial.
• Some have again become superficially bilateral
  (skeletal organ systems still pentaradial).
• Sea cucumbers.
• A few sea urchins.
• No well defined head or brain.

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Phylum Echinodermata - Deuterostomes

• Echinoderms have a true coelom with deuterostome
  development.
• Radial, indeterminate cleavage.
• Enterocoelous the mesoderm lined coelom
  develops from outpocketing of the primitive gut.
• Formation of the mouth at the end of the embryo
  opposite the blastopore.

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Water Vascular System

• Echinoderms have a water vascular system derived
  from part of the coelom.
• A system of canals and specialized tube feet that
  functions in
• Locomotion
• Food gathering
• Respiration
• Excretion

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Water Vascular System

• The water vascular system opens to the outside
  through small pores in the madreporite.

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Water Vascular System

• Canals of the water vascular system lead to the
  tube feet.
• Tube feet may have suckers, allowing the
  echinoderm to move while remaining firmly
  attached to the substrate important in areas
  with lots of wave action.

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Endoskeleton

• Echinoderms have an endoskeleton of calcareous
  ossicles often with spines.
• Endoskeleton is covered by an epidermis.
• Some have a very substantial endoskeleton (sea
  urchins), others have only a few scattered dermal
  ossicles (some sea cucumbers).

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Development

• Eggs (which may be brooded or laid as benthic egg
  masses) hatch into bilateral, free-swimming
  larvae.
• The type of larva is specific to each echinoderm
  class.

• Class Asteroidea
• Bipinnaria
• Brachiolaria
• Class Ophiuroidea
• Ophiopluteus
• Class Echinoidea
• Echinopluteus
• Class Holothuroidea
• Auricularia
• Class Crinoidea
• Doliolaria

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Development

• Metamorphosis involves a reorganization into a
  radial juvenile.
• Left/right becomes oral/aboral.

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Class Asteroidea

• Class Asteroidea includes sea stars.
• Common on rocky shores and coral reefs, some
  found on sandy substrates.

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Class Asteroidea

• Sea stars have arms (rays) arranged around a
  central disc.
• The body is flattened, flexible, and covered with
  a ciliated, pigmented epidermis.

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Class Asteroidea

• The mouth is on the underside of the sea star.
• Ambulacral grooves stretch out from the mouth
  along each ray.
• Tube feet border each groove.

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Class Asteroidea

• The aboral surface is often rough and spiny.
• Around the base of each spine there are
  pincerlike pedicellariae that keep the surface
  free of debris and sometimes help with food
  capture.

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Class Asteroidea

• Skin gills are soft epidermis covered projections
  of the coelom that extend between ossicles and
  serve a respiratory function.

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Class Asteroidea

• The lower part of the stomach can be everted
  through the mouth during feeding.

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Class Asteroidea

• The upper part of the stomach connects to a pair
  of digestive glands (pyloric ceca) in each arm.

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Class Asteroidea - Feeding

• Most sea stars are carnivorous feeding on
  molluscs, crustaceans, polychaetes, echinoderms,
  other inverts sometimes small fish.

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Class Asteroidea - Reproduction

• Most sea stars have separate sexes with a pair of
  gonads in each ray.
• Fertilization is external.

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Class Asteroidea - Regeneration

• Echinoderms can regenerate lost parts.
• Sea stars can readily replace an arm if it is
  lost.
• This may take several months.
• They can also cast off an injured arm.

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Class Asteroidea - Regeneration

• Some species can even regenerate an entire
  individual from a broken off arm.
• Usually, a small piece of the central disc must
  be included.
• Linckia can regenerate a whole new individual
  from a broken arm with no central disc attached.

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Concentricycloidea

• The two species of sea daisies were described for
  the first time in 1986.
• They are tiny (lt 1 cm), have no arms and the tube
  feet are arranged around the periphery of the
  disc.
• Once considered a separate class, they are highly
  derived sea stars.

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Class Ophiuroidea

• Brittle stars (Class Ophiuroidea) are the largest
  group of echinoderms.
• Abundant in all benthic marine environments
  even the abyssal sea bottom.
• Brittle stars have very slender arms.

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Class Ophiuroidea

• No pedicillariae or skin gills.
• Madreporite is on the oral surface.
• Tube feet have no suckers, their primary function
  is to aid in feeding.

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Class Ophiuroidea

• Brittle stars move using their arms rather than
  tube feet.

http//youtu.be/BWOdssnzsMY
http//youtu.be/4Texm2eTmSc
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Class Echinoidea

• Class Echinoidea includes sea urchins and sand
  dollars.

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Class Echinoidea

• The endoskeleton is well developed in echinoids.
• Dermal ossicles have become close-fitting plates
  that form the test.

http//www.jaxshells.org/test.htm
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Class Echinoidea

• Echinoids lack arms, but still show the
  pentamerous plan in the five ambulacral areas
  with pores in the test for the tube feet.

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Class Echinoidea

• Most echinoids are regular having a
  hemispherical shape, radial symmetry, and medium
  to long spines.
• Regular urchins move using their tube feet with
  some help from spines.

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Class Echinoidea

• Irregular echinoids include the sand dollars
  and heart urchins that include some species that
  have become bilateral.
• Spines are usually short and are used in
  locomotion.

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Class Echinoidea

• Some urchins have very reduced tests, and bright
  coloration.
• The pedicellariae in these species contain
  painful toxins.

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Class Echinoidea

• Echinoids live in all seas from the intertidal to
  the deep sea.
• Urchins usually prefer rocky substrate, while
  sand dollars and heart urchins like to burrow
  into sandy substrate.

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Class Echinoidea

• Echinoids have a complex chewing mechanism called
  Aristotles lantern.
• Teeth are attached here.
• Sea urchins are usually omnivorous feeding mostly
  on algae.

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Class Echinoidea

• Sand dollars use their short spines to move sand
  its organic contents to the sides, the food
  particles drop between the spines, and ciliated
  tracts on the oral side carry the particles to
  the mouth.

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Class Holothuroidea

• Sea cucumbers (Class Holothuroidea) are elongated
  along the oral/aboral axis.
• Bilateral
• Ossicles are greatly reduced in most species.

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Class Holothuroidea

• The body wall is usually leathery with tiny
  ossicles embedded in it, but can be very thin.

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Class Holothuroidea

• Oral tentacles are modified tube feet located
  around the mouth.
• Food particles are gathered by the oral
  tentacles.
• Tentacles are put into the pharynx one by one so
  food can be sucked off.

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Class Holothuroidea

• Sea cucumbers move using ventral tube feet and
  waves of contraction along the muscular body wall.

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Class Holothuroidea

• Sea cucumbers have a very unusual defense
  mechanism
• They are able to cast out part of their viscera.
• The lost parts regenerate.
• Some have organs of Cuvier that can be expelled
  in the direction of an enemy.
• These tubules become long and sticky, sometimes
  containing toxins.

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Class Crinoidea

• Crinoids include sea lilies and feather stars.
• At metamorphosis, juveniles become sessile and
  stalked.
• Adults are free-moving in some species.
• Long, many branched arms.

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Class Crinoidea

• Crinoids use their tube feet and mucus nets to
  feed on small organisms that are passed to their
  ciliated ambulacral grooves.

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Phylogeny

• Echinoderms are probably derived from bilateral
  ancestors.
• Pentaradial symmetry may have been an adaptation
  to a sessile existence.
• Some forms then become mobile.
• Some mobile forms are secondarily bilateral.

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Phylum Hemichordata

• Hemichordates (acorn worms) are marine animals
  that have gill slits and a rudimentary notochord
  however, the notochord is not homologous with
  the notochord in vertebrates.

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Phylum Hemichordata

• Vermiform bottom dwellers, usually in shallow
  water.
• Some are colonial living in secreted tubes.

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Phylum Hemichordata

• Hemichordates are deuterostomes with radial
  indeterminate cleavage and enterocoelous coelom
  development.
• Larvae are similar to those of echinoderms.

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Phylum Hemichordata

• A tubular dorsal nerve cord in the collar zone of
  acorn worms seems to be homologous to that in
  chordates.
• Gill slits in the pharynx serve for filter
  feeding and secondarily for breathing another
  characteristic found in chordates.

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Phylogeny

• Hemichordates share characteristics with
  echinoderms
• Early embryogenesis
• Similar larvae
• And Chordates
• Gill slits
• Dorsal hollow nerve cord