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Phylum
Echinodermata
Introduction
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Echinodermata are all marine,
triploblastic unsegmented
coelomates
Phylum has 3 unique features:
pentagonal symmetry (bilateral in
larvae)
 calcite spicules embedded in the
skin, often partly fused
 Tube feet (podia)
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Affinities
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The only connected phylum is
our own, the chordates - based
on embryological evidence.
An unhurried phylum..
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No echinoderm moves fast, apart
from a very few deep sea
holothurids which swim actively
Crinoids are sessile, the others
crawl at a rate of mm / minute
During one Antarctic marine survey
a starfish was tagged. A year later
the same animal was in the same
exact spot, having apparently done
nothing at all!
Anatomical basics:
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There is no cephalization
There is a meaningful gradient in
all echinoderm bodies: one surface
has the mouth and tube feet
(ORAL or AMBULACRAL), while
one does not (ABORAL)
The anus is often, but not always,
aboral.
Originally…
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The ancestral echinoderm was a
sessile filter-feeder, extending its
oral surface upwards to capture
food
This sedentary design has evolved
into motile forms where the feeding
surface faces downwards
Functional groups 1:
nerves
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Echinoderms have a diffuse
nervous system with no “brain”
There is a 5-radial circum-oral
nerve ring, and a superficial net
running close to ectoderm
Hydraulics
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These are far more complex than
the nervous system!
Main hydraulic systems are
derived from the coelom, although
separate sections of the coelom
also surround viscera
The podia are operated by a
hydraulic system called the watervascular system
5-radial layout
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Many organ systems in the
echinoderms follow the same basic
structure as the water-vascular and
nervous systems: a 5-radial circumoral ring
These rings give rise to 5 radial
branches (canals in the case of the
WVS)
A few asteroids have 7, 10, 11 arms in which case 7,10, 11 radial branches
Hydraulics, contd.
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Each radial canal of the WVS
supplies water to tube feet, each
with its ampulla
There is one asymmetric
element: a single tube (the
“stone canal”) running from the
oral WVS ring to the outside via
the madreporite
Surface features
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Echinoderm skin has several
distinctive sets of organs
protruding from their skin:
 Tube
feet (podia)
 Spines
 Pedicillaria
Tube feet..
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Podia are not scattered haphazardly
over the body surface
They lie in 10 rows (5 pairs), the
ambulacral grooves
Each tube foot + its ampulla is isolated
from the WVS by a valve
Tube feet vary - starfish have muscular
suction cups, other forms have sticky
tips.
Crinoids are different - primitive
Tube feet..
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Originally began as outgrowths of
the WVS. In crinoids and
ophiuroids these remain essentially
as tentacles.
In other radiations, notably
asteroids, these have evolved a
highly specialised suction cup used
for locomotion and prey capture.
Tube feet..
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Have retractor muscles and can
bend, but no extensors
To extend, muscles around the
ampulla contract
Each podium has a nervous arc to
its branch of the hyponeural
system
Role of WVS
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Hydraulics
Respiration - O2 is exchanged
between ampulla and perivisceral
coelomic fluid
Probably (?) this was the ancestral
function of the WVS, with tubes +
podia lining arms to exploit ciliary
current already used in food
collection
Pedicillaria
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…Are defensive organs, assumed
to protect against encrusting
organisms
Are active, independent local
effector units able to inject toxins
on contact
Madreporite
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Allows pressure equalization and
top up water supply to the WVS
Is absent in crinoids
Gonads
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Lie as 10 (2N) paired structures
at the base of ambulacral
grooves.
Sexes are separate, and
discharge gametes into the sea
water
Gonads can be large - echinoid
gonads almost fill the test, and
can be eaten as a delicacy.
Phylum Echinodermata
Living forms only
Ophiuroidea
Brittlestars
Crinoidea
Crinoids - feather stars
Echinoidea
Sea urchins
Asteroidea
starfish
Holothuridea
sea cucumbers
Concentricycloidea
Sadly...
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Of the 13 classes of echinoderms
known, 7 are extinct.
Echinoderms were dominant forms
in Carboniferous seas, but have
suffered a long-term decline in
phyletic richness
Crinoidea
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Feather stars & Sea lilies
Abyssal filter feeders
5000 fossil spp, 620 living
Crinoidea
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Body made of ossicles
10 arms have podia (no ampullae)
feeding particles to the mouth.
Arms can move
Mouth and anus are both on oral
side (!)
Asteroidea
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“Starfish”
Active predators
feed on bivalves
 use suction cups to pull open the
shells with forces of up to 5kg
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The stomach is eversible, and can be
partially inserted inside prey’s shell
(enzymes but no toxins)
Echinoidea
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Recipe: take a starfish and roll
its 5 arms together into a ball,
then fuse and calcify with an
external armor
The armor is called the test
Very small aboral surface
Echinoidea
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Herbivores, preferring macro-algae
They can be highly effective grazers,
creating “urchin barrens” devoid of
algae
The mouthparts are unique, known as
Aristotle’s Lantern.
5 continually growing chisel teeth
 Each tooth with 8 supporting skeletal
pieces
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Irregulars
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All are sand burrowing
Heart urchin Echinocardium has no
lantern;
Sand-dollars (Clypeaster) are more
flattened with a lantern
Noli tangere
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Many echinoids have wickedly
sharp spines, which break off in
your skin.
Only a few fish, trigger fish
attack long-spined species
Spines are under muscular
control, and can be used to
move
Noli tangere
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Very few echinoids are lethal to
touch - their pedicillaria inject a
neurotoxin
Toxopneustes is feared by pearl
divers
Ophiuridae - brittle stars
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Have arms sharply demarcated
from the body disc.
The internal structure of the arms
involves interlocking internal
ossicles, confusingly called
vertebrae
Are primarily detrital or filter
feeders, raising their arms in a
current to capture particulates
Holothuridae- Sea
Cucumbers
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They have no calcitic skeleton, except
for spicules embedded in a leathery skin
Most are immobile, and lie on the sea
bed rolling back and forth with the swell.
Some have limited mobility using their
tube feet.
Despite retaining 5-radiate anatomy, they
have re-evolved bilateral symmetry along
their long axis (the oral-aboral)
Holothuridae
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They mainly feed on detritus
Oxygen exchange is performed
using gills inside their anus
They have 2 odd defensive
strategies:
 Squirting a sticky goo
 Voiding their entire intestines