Phylum

Echinoderma

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Introduction



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)

Affinities



The only connected phylum
is our own, the chordates -
based on embryological
evidence.

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!

Anatomical

basics:



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…



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



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



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
water-vascular system

5-radial layout



Many organ systems in the
echinoderms follow the same
basic structure as the water-
vascular and nervous systems: a
5-radial circum-oral 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.



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



Echinoderm skin has several
distinctive sets of organs
protruding from their skin:



Tube feet (podia)



Spines



Pedicillaria

Tube feet..



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..



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..



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



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



…Are defensive organs,
assumed to protect against
encrusting organisms



Are active, independent local
effector units able to inject
toxins on contact

Madreporite



Allows pressure equalization
and top up water supply to the
WVS



Is absent in crinoids

Gonads



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.

Sadly...



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



Feather stars & Sea lilies



Abyssal filter feeders



5000 fossil spp, 620 living

Crinoidea



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



“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)

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

Echinoidea



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

Irregulars



All are sand burrowing



Heart urchin Echinocardium
has no lantern;



Sand-dollars (Clypeaster) are
more flattened with a lantern

Noli tangere



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



Very few echinoids are
lethal to touch - their
pedicillaria inject a
neurotoxin



Toxopneustes is feared by
pearl divers

Ophiuridae -

brittle stars



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



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



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