Phylum Echinodermata

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Phylum Echinodermata

• The echinoderms (hedgehog skin) are a very
  unusual group that includes about 7000 living
  species.
• Members include starfish, brittle stars, sea
  urchins, sea cucumbers, and sea lilies or feather
  stars.
• They are deuterostomes (as are chordates), but
  have secondarily evolved radial symmetry from
  bilateral symmetry (they still have bilaterally
  symmetrical larvae).

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Characteristics of the Echinodermata

• Exclusively a marine group. They cannot
  osmoregulate so rarely occur even in brackish
  water.
• The body is not segmented, but shows pentaradial
  symmetry.
• There is no head or brain and the nervous system
  is relatively simple.

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Characteristics of the Echinodermata

• They possess an endoskeleton of dermal calcareous
  ossicles, which are connected together by
  connective tissue.
• Possess a unique water vascular system that
  consists of a series of canals that extend from
  the body surface as tube feet.
• These tube feet are tentacle-like and enable the
  animal to move. In some species movement of the
  arms or spines contributes to locomotion too.

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Classes of Echinoderms

• There are a total of five classes of echinoderms
  and about 7300 species.
• Class Asteroidea sea stars or starfishes
• Class Ophiuroidea Brittle stars
• Class Echinoidea Sea Urchins, Sand dollars
• Class Holothuroidea Sea cucumbers
• Class Crinoidea Sea Lilies and Feather stars.

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Class Asteroidea

• Star fish are the most familiar of the
  echinoderms and demonstrate the characteristics
  of the group well.
• Typically, they have 5 tapering arms, which merge
  gradually with the central disc.
• The mouth is found on the oral surface, the
  opposite side of the animal being referred to as
  the aboral surface.

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Cushion seastar (Asteroidea)
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Class Asteroidea

• The aboral surface of starfish is usually rough
  to the touch and has spines (although these may
  be flattened so the animal appears smooth.).
• Around the base of the spines are small
  pincerlike structures called pedicellariae, which
  are under muscular control and help to remove
  debris and protect the animals papulae (skin
  gills) projections of the coelomic cavity that
  increase the surface area for gas exchange.

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Pedicellariae
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Water vascular system

• Grooves called ambulacral grooves radiate out
  from the mouth on the oral surface. Tube feet
  project from the grooves.
• These tube feet are connected to the water
  vascular system, which is a unique invention of
  the echinoderms.

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Water vascular system

• The water vascular system is a compartment of the
  coelom and is a system of canals and tube feet.
• The water vascular system uses hydraulic pressure
  to extend, move and contract the tube feet
  enabling the starfish to move and feed.

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Water vascular system

• The water vascular system opens to the outside
  via small pores in a structure called the
  madreporite on the aboral surface.
• The madreporite connects to a ring canal and from
  this ring canal, radial canals extend down the
  abulacral groove of each arm.

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Water vascular system

• From each of the radial canals smaller lateral
  canals branch off, which connect to the tube
  feet.
• Each tube foot is a hollow, muscle-lined tube,
  which has a bulb at one end (on the inside of the
  arm) and a sucker at the other end which
  protrudes from the animal.

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Water vascular system

• The tube feet use hydraulics to work.
• Each tube foot contains a lot of connective
  tissue in its wall that maintains the tube at a
  fairly constant diameter.
• Contraction of muscles in the ampulla forces
  fluid into the tube foot which extends (a valve
  prevents fluid flowing back into the lateral
  canal).

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Water vascular system

• To withdraw the foot, muscles in the tube foot
  contract which pushes fluid back into the
  ampulla.
• The tube foot can bend to one side as a result of
  the contraction of muscles on one or other side
  of the tube foot.
• In addition, small muscles at the end of the tube
  foot can contract to raise the center of the tube
  foot and create a suction cup.

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Water vascular system

• As a result of the combination of hydraulics and
  muscular control the tube feet can be moved with
  a high degree of control and can also exert a
  strong pull.
• This enables the starfish to move up hard
  vertical surfaces and open up prey such as
  bivalves.
• On sediments suction doesnt work well and under
  these conditions the tube feet are used more like
  legs.

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Feeding and digestion

• The mouth of a sea star opens into a two-part
  stomach in the central disk of the animal.
• The lower cardiac stomach can be everted and this
  ability is important in sea stars ability to
  consume certain prey.

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Feeding and digestion

• The cardiac stomach is connected to a pyloric
  stomach above it which in turn connects to
  digestive cecae in the arms where most
  extracellular digestion takes place.
• A short intestine leads from the pyloric stomach
  to the anus on the aboral side.

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Feeding and digestion

• Most sea stars are quite unselective carnivores
  and feed on molluscs, crustaceans, annelids,
  other invertebrates and other echinoderms.
• Some sea stars are major predators of
  economically important species of bivalve such as
  mussels and oysters.

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Feeding and digestion

• When a sea star attacks a bivalve it wraps itself
  around it and uses its tube feet to try to pull
  the two shells apart.
• The adductor muscles of the bivalve keep them
  closed, but the sea star can maintain its pull
  for long periods and eventually (after about half
  an hour) the adductor muscles of the bivalve
  become fatigued and the shells separate a little.

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Feeding and digestion

• Once a gap opens the sea star everts its soft
  flexible stomach into the gap and begins to
  secrete digestive enzymes that slowly break down
  the soft tissues of the bivalve

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Reproduction and development

• Most sea stars have separate sexes and
  fertilization is external.
• Eggs are brooded by some species, but in most
  cases the eggs are released to develop into
  larvae.
• Early development is typical of deuterostomes and
  the free swimming bipinnaria larva is bilaterally
  symetrical.

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Reproduction and development

• Late in development, however, the larva follows a
  path quite different from other deuterostomes.
• The bipinnaria larva transforms into a
  brachiolaria by growing three adhesive arms and a
  sucker at its anterior end.
• It then attaches to a substrate and undergoes
  metamorphosis.

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Reproduction and development

• During metamorphosis the larva is radically
  altered.
• The anterioposterior axis is lost and what was
  the left side becomes the oral surface and what
  was the right side becomes the aboral surface.

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Reproduction and development

• The existing mouth and anus disappear and new
  ones are produced in the appropriate locations.
• Finally short arms and podia develop and the
  animal detaches from its stalk to become a young
  starfish.

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Regeneration

• Echinoderms have the capacity to regenerate lost
  parts. For example, a sea star can regenerate a
  leg (or even all of its legs) if it is lost).
• If an arm that is removed contains at least one
  fifth of the central disk a new individual will
  develop from it.

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Class Ophiuroidea

• The Ophiuroidea includes the brittle stars and
  basket stars.
• This is the largest class of echinoderms with
  more than 2000 living species and it probably is
  also the most abundant group.
• They are very common in all kinds of benthic
  marine environments and abound on the abyssal sea
  floor in many places.

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Class Ophiuroidea

• Although similar to sea stars in having five
  arms, the brittle stars can be readily
  distinguished by their proportionally much longer
  and thinner arms and by the clear separation
  between the arms and the central disk.

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Brittle star (Ophiuroidea)
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Class Ophiuroidea

• Brittle stars move in a different manner to
  starfish. Their ambulacral grooves are covered
  over with interlocking ossicles.
• They possess tube feet which play a role in
  feeding, but dont contribute to movement.
• Movement instead occurs by movement of the
  jointed arms.

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Class Ophiuroidea

• The mouth is surrounded by five moveable plates
  that act as jaws.
• Brittle stars lack intestines and an anus and
  waste exits via the mouth.
• Brittle stars feed on small particles that they
  glean from the sea bottom or particles they sieve
  out of the water with mucus strands attached to
  spines on their arms

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Class Echinoidea

• The Echinoidea include the familiar sea urchins,
  sand dollars, and heart urchins.
• They have a compact body that is enclosed in a
  calcareous shell (or test), which is formed from
  dermal ossicles that have become a series of 10
  double rows of close-fitting plates.
• These plates also bear moveable, stiff, often
  long spines.

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Sea urchins
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Sand dollar
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Class Echinoidea

• The sea urchins are considered to be regular
  echinoids because they are hemispherical and
  radially symmetrical.
• However, the sand dollars and heart urchins are
  considered irregular because they have become
  secondarily bilaterally symmetrical.

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Class Echinoidea

• In sand dollars the anus and mouth are located on
  the oral side with the mouth near the middle, but
  the anus shifted near the margin, so that an
  anterioposterior axis is recognizable.
• This is even more apparent in the heart urchins
  where the mouth is moved towards the anterior end
  and the anus (periproct) near the posterior end.

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Class Echinoidea

• Echinoids are widely distributed in all seas.
  Sea urchins are most typically found on rocky
  substrates, but sand dollars and heart urchins
  prefer sandy substrates.
• Sea urchins feed by grazing on algae growing on
  rocks and they do so using a complex structure
  called an Aristotles lantern.

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Aristotles Lantern

• The Aristotles lantern is a chewing mechanism
  that can be raised and lowered by muscles inside
  the urchin.
• The lantern consists of five plates connected to
  each other with connective tissue in a vase-like
  arrangement and each plate has a tooth on the
  end.

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Aristotles Lantern

• The lantern can be lowered to scrape off algae
  and then retracted inside using paired muscles.
• The Aristotles lantern connects directly to the
  esophagus, which connects in turn to the rest of
  the gut.

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Class Holothuroidea

• The Holothuroidea or sea cucumbers are a quite
  odd group whose members at first glance dont
  seem that similar to the other echinoderms.
• Sea cucumbers are very elongated along the
  oral-aboral axis and have an array of oral
  tentacles (which are modified tube feet) around
  the mouth.

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Sea cucumber
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Class Holothuroidea

• Holothuroideans have a leathery body as the
  ossicles are small.
• Because they lie on one side, sea cucumbers
  typically have well developed tube feet on only
  the three ambulacra in contact with the
  substrate.
• The other two ambulacra have less well developed
  tube feet, which are not used for movement, but
  may play a sensory role.

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Class Holothuroidea

• Sea cucumbers are sluggish and dont move
  quickly.
• They feed either on deposits, which they collect
  as they crawl along the sediment or are filter
  feeders that filter small particles from the
  water.
• Deposit feeding sea cucumbers are a successful
  group and make up as much as 90 of the biomass
  on the surface of the deep-sea.

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Class Holothuroidea

• An unusual feature of sea cucumbers is their
  respiratory tree, which is a many branched
  internal respiratory structure that connects to
  the cloaca.
• Water is pumped in and out of the tree by the
  muscular cloaca and the tree itself and gas
  exchange takes place.

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Class Holothuroidea

• Attached to the respiratory tree are structures
  called Cuverian tubules. These are used in
  defense and are discharged out of the animal if
  it is disturbed.
• The tubules are long, sticky and sometimes toxic
  and can entangle an attacker. Some species also
  may discharge their digestive tract, respiratory
  tree or gonads. These structures can be
  regenerated.

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Class Crinoidea

• Includes the sea lilies and feather stars, which
  have an extensive fossil record and once were
  much more abundant in the seas.
• The crinoids have a very feathery appearance that
  results from the branching of their five arms to
  produce many more arms each of which has many
  lateral pinnules.

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Class Crinoidea

• Crionoids occur most commonly in deep water, but
  there are shallow water species too.
• Not surprisingly with their appearance they
  filter feed and the food is carried down the
  ambulacral grooves to the mouth by cilia and tube
  feet.

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Class Crinoidea

• Crinoids have a similar water vascular system to
  other echinoderms and have only simple sense
  organs. They lack spines, a madreporite and
  pedicellariae.
• Most living species are not more than 12 inches
  long, but some fossil species had stalks over 75
  feet long.

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Phylum Hemichordata

• The hemichordates are deuterostome marine animals
  that were previously classified as a subphylum of
  the chordates because they possess a dorsal nerve
  cord, gills slits and a rudimentary notochord.
• However, there is now consensus that the
  hemichordate notochord is not homologous with
  that of the chordates. Thus, they have been
  classified in their own phylum.

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Phylum Hemichordata

• The hemichordates are wormlike organisms that
  live on the bottom and are usually found in
  shallow waters.
• There are two classes the Enteropneusta (the
  acorn worms) and the Pterobranchia.

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Acorn worms

• The acorn worms are deposit or suspension feeders
  that use their large proboscis to trap food in
  mucus that is then transported by cilia to the
  mouth.

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Saccoglossus an acorn worm (Hemichordata class
Enteropneusta)
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Acorn worms

• The larva of acorn worms is called a tornaria
  and closely resembles the bipinnaria larva of
  echinoderms, which supports their classification
  as close relatives of the echinoderms.

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(Hemichordate)
(Echinoderm)
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Pterobranchia

• The pterobranchs are small colonial animals
  (1-7mm in length).
• They live in tubes which they produce and can
  freely move about in filter feeding using their
  arms, which are covered in tentacles.
• They lack a dorsal nerve cord, but one genus has
  a pair of gill slits.

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Cephalodiscus a pterobranch hemichordate
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Taxonomy of hemichordates

• Hemichordates were once classified among the
  chordates on the basis of their gill slits and
  dorsal nerve cord.
• More recently molecular sequence work that has
  examined Hox genes and rRNA places the
  henichordates in a clade with the echinoderms.
  This is supported by the similarities in larvae.
  

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Taxonomy of hemichordates

• In addition, research on an extinct goup called
  the carpoids has found gill slits which suggests
  that gill slits are ancestral for all
  deuterostomes.

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