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ECHINODERMS:
Phylum: Echinodermata
Class: Echinoidea
(sea urchins)
Order:Regular Irregular
Crinoidea
(sea lilies)
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.
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.
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
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.
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.
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).
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.
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.
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.
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.
Echinoids’ Mode Of Life
 Varies depending on
whether the echinoid
is regular or irregular.
 All are benthonic, can
move and are
gregarious.
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.
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.
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.
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.
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.
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 Darwin’s
theory of evolution.