Phylum
Echinodermata
Characteristics


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)

Characteristics

There is no cephalization


Which explains why Patrick isn’t the smartest
creature in bikini bottom!
There is a meaningful gradient in all
echinoderm bodies:
 one
surface has the mouth and tube
feet (oral or ambulacra),
 while one does not (aboral)

The anus is often, but not always,
aboral.
An unhurried phylum..



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!
Originally…


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
Characteristics

The only connected phylum is our
own, the chordates - based on
embryological evidence.
 Look
at Evolutionary Tree in
textbook

Deuterosome
 Radial
cleavage
 Anus forms first

First species to evolve with this
type of development
Characteristics


The aboral surface is rough and
spiny
Around the base of the spines are
pedicillaria: tiny jaws used to keep
the body surface free of debris,
protection and aide in food capture
Water Vascular System



Primary function is locomotion and food
gathering, in addition to respiration and
excretion
From the aboral surface is a
madreporite (allows pressure
equalization and top up water supply)
which is the only asymmetrical tube in
the system
It connects to an inner Ring Canal in
which branches into 5’s lateral canals
(one down each of the star fishes arms)
Water Vascular System

From the Lateral canals is many
ampulla (muscular sacs) which
connected to the suckers on the
outer surface of the starfish
 Some

lack suckers
By filling the ampulla and coelom
with liquids and applying pressure,
allows arms to stiffen creating a
hydraulic system.
Tube feet





Podia (tube feet) are not scattered over
the body surface
They lie in 10 rows (5 pairs), the
ambulacral grooves
Each tube foot + its ampulla is isolated
from the water vascular system by a
valve
Tube feet vary - starfish have muscular
suction cups, other forms have sticky
tips.
Crinoids are different - primitive
Reproduction



Gonads lie as 10 (2N) paired
structures at the base of
ambulacral grooves.
External reproduction
Sexes are separate, and discharge
gametes into the sea water
Development


Larval Stage- free-swimming, bilateral
(some with direct development)
Metamorphosis




Metamorphosis involves a dramatic
reorganization of a bilateral larva into a radial
juvenile
Larval mouth and anus disappear and a new
mouth and anus will form.
What was the left side becomes the oral
surface
What was the right side becomes the aboral
surface
Phylum
Echinodermata
EVOLUTION
Sadly...


Of the 13 classes of echinoderms
known, 8 are extinct.
Echinoderms were dominant forms
in Carboniferous seas, but have
suffered a long-term decline in
phyletic richness
Phylum Echinodermata
Living forms only
Ophiuroidea
Brittlestars
Crinoidea
Crinoids - feather stars
Echinoidea
Sea urchins
Asteroidea
starfish
Holothuridea
sea cucumbers
Concentricycloidea
Classes





Class Crinoidea
Class Asteroidea
Class Ophiuroidea
Class Echinoidea
Class Holothuroidea
Class Crinoidea







Feather stars & Sea lilies
Abyssal filter feeders
5000 fossil spp, 620 living
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
Class Asteroidea


“Starfish”
Active predators
feed on bivalves
 use suction cups to pull open the
shells with forces of up to 5kg


The stomach is eversible, and
can be partially inserted inside
prey’s shell (enzymes but no
toxins)
Class Ophiuroidea




Brittle Stars
Have arms sharply
demarcated from the body
disc.
The internal structure of the
arms involves interlocking
internal ossicles (also called
vertebrae)
Are primarily filter feeders,
raising their arms in a
current to capture
particulates
Class Echinoidea



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
Class Echinoidea




Sea Urchins
Herbivores, preferring macroalgae
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

Class Holothuridae




Sea Cucumber
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)
Class Holothuridae



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