ECHINODERMS:

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ECHINODERMS

• Phylum Echinodermata
•  
• Class Echinoidea Crinoidea
• (sea urchins) (sea lilies)
•  
• OrderRegular Irregular

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PHYLUM ECHINODERMATA

• Appeared in the Lower Palaeozoic and have forms
  still living today.
• They are marine with a calcareous skeleton.
• Ideally 5 fold symmetry.
• Examples include sea urchins, starfish, brittle
  stars, sea cucumbers and crinoids.
• The skeleton (TEST) is internal and is
  constructed of individual calcite plates.
• There is a hydraulic system of tube feet.

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CLASS ECHINOIDEA

• They are exclusively marine in shallow depths to
  the abyssal planes.
•  MORPHOLOGY
•  ? Draw the diagram on page 175 (Black) Figs. a
  and j.
• They have a hard shell which when alive is
  covered by a very thin skin and therefore they
  have an ENDOSKELETON.

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Echinoid Morphology 2

• The skeleton (TEST) is made of calcite with tiny
  interlocking plates which protect and enclose
  most of the soft parts inside.
•  The test is usually hemispherical, the
  interlocking plates are arranged in 10 double
  columns radiating out from the top of the upper
  surface (CORONA).
•  See page 175 Fig. a.
•  There are two types of plate
• AMBULACRUM
• INTERAMBULACRUM

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Echinoids Morphology 3

•  Ambulacrum These occur where the TUBE FEET are
  positioned.
•  These feet are connected to the WATER VASCULAR
  SYSTEM (system of hydraulic tubes) through which
  water is circulated around the body and can be
  used to extend the tentacles through the test and
  can act like feet.
•  

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Echinoid Morphology 4

• Towards the top (APEX) of the test is the APICAL
  SYSTEM which is made up of about 10 small plates
  that are interconnected.
•         Page 175 Fig. 106 c.
• One plate has a special function it is porous
  and allows sea water into the body MADREPORITE.
•   This water then passes through the RADIAL
  CANALS and into the tube feet.

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ORDERS OF ECHINOIDS

• Echinoidea are divided into 2 orders which can be
  achieved by looking at their symmetry
• REGULAR ECHINOIDS
• They are usually circular when viewed from above.
• They show a 5-fold symmetry. Therefore they have
  a regular pattern.
• The apical system is situated on the top and
  contains the anus in the centre surrounded by the
  PERIPROCT (membrane).

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Regular Echinoids 2

• The mouth is situated on the underside (ORAL
  SURFACE) usually in the centre.
•  JAWS are present although they are rarely
  preserved.
• The upper surface is called the ABORAL SURFACE.

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IRREGULAR ECHINOIDS

•  ? Look at Page 190 (Black) Micraster and copy
  the diagram.
•   These are not circular but are either
  flattened or heart shaped.
•  They still have 5 rows of ambulacrum and
  interambulacrum plates but instead of 5-fold
  symmetry they show a bilateral symmetry.

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Irregular Echinoids 2

• The ANUS is not enclosed within the apical
  system.
• Instead it lies either
• 1) On the aboral side half way up the side
  (Posterior). Sometimes in a groove.
• 2) On the oral surface towards the posterior.
• The MOUTH is found on the oral surface either
• 1) In the centre with jaws.
• 2) Closer to the front (anterior) without jaws.
• Therefore it is easier to define anterior and
  posterior.

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Irregular Echinoids 3

• Frequently the two rows of pores within the
  double ambulacrum plate can diverge from each
  other and then converge lower down the test
  forming a distinctive pattern called PETALS or
  PETALOID.
• Sometimes the posterior interambulacrum area can
  extend down across the oral surface, this usually
  occurs when the mouth is posterior closer to the
  anterior end.
• This forms a flatish ridge on the oral surface
  called the PLASTRON.
•  This may project like a lip across part of the
  mouth LABRUM.

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Echinoids Mode Of Life

• Varies depending on whether the echinoid is
  regular or irregular.
• All are benthonic, can move and are gregarious.

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Regular Echinoids

• They are usually mobile, moving about looking for
  food and protection.
• Many are capable of living on hard rocks anchor
  themselves to the rocks via tube feet even in
  relatively shallow water.
• Common between the sub littoral zone down to 100
  m.
• Can also use the tube feet to climb steep rock
  surfaces.
• On sand they use their spines to support them and
  move themselves using the spines on the oral
  surface and low down on the aboral.
• Could move in any direction.
• They eat sea weed but also partly carnivorous
  bryozoa and sponges in particular.
• Have strong jaws e.g. Echinus lives on rocks.

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Irregular Echinoids Mode of life

• A) FLATTENED TEST
• ? Draw diagram from page 185 Black
• Clypeaster lived partially or completely buried
  in loose sediment and moved forward by moving
  spines to plough through soft sediment.
• The tube feet extract organic matter from
  sediment and transfer to food tubes.
• Lives in shallow water 0.5 - 5 m.

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Irregular Echinoids 2

• B) HEART SHAPED
• Micraster and Echinocardium which could be
  completely buried.
• Common down to 50 m but can survive down to 200 m
  below sea level.
• Lived in burrows of soft sediment (Micraster in
  fine lime mud).
• ? Draw diagram from page 183 Black.
•  Burrows forwards using spines and tube feet
  (Mucus can be secreted to help stabilise the
  sediment to stop collapsing.

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Irregular Echinoids 3

• Sand etc. is pushed aside and backwards.
• Organic matter is extracted from the sediment and
  the waste disposed behind.
• Some food is also obtained from the sea water via
  a FUNNEL which extends from the burrow.
• The tube feet in the upper areas extend out of
  the burrow.
• Water is drawn into the animal and CILIA help
  waft it into the tube feet respiratory system.
•  All are gregarious.

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Echinoid history

• Upper Ordovician to Recent
• Began in the Upper Ordovician but only a small
  number.
• In the Carboniferous the numbers peaked briefly
  but reduced during the Permian.
• During the Mesozoic (Triassic) the numbers
  increased again with new species due to a major
  adaptive radiation after the Permian extinction
  provided new niches.
• They are very rarely found as Palaeozoic fossils
  as they did not burrow and plates of test not
  well fused therefore broke up.
• Those preserved are usually found in limestone.

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Echinoid History 2

• Irregular appear in the Upper Jurassic and
  increase quickly in numbers.
• They increase so quickly because they were more
  efficient food grazers and had improved
  sanitation with anus removed from the apical
  system.
• Common in limestone particularly chalk.
• Still abundant today.
• Micraster was a very important fossil as it
  evolved quite quickly and palaeontologists were
  able to show it changing its mouth and anus
  positions over time.
• This added proof to Darwins theory of evolution.