Phylum Echinodermata - Prof. L. D. Amarasinghe

Phylum Echinodermata
(Greek: Echinos=Hedgehog(Spiny) skinned)
The Echinodermata along with Chordata,
Lophophorates and Hemichordata (acorn worms)
are deuterostomes.
Cladogram showing Deuterostome Phyla of the Animal KIngdom
Deuterostomia Branch
Blastopore of gastrula becomes anus;
mouth forms secondarily
6,500 living species
and
13,000 fossil records
Echinodermes are exclusively marine.
As they are unable to osmoregulate, they
cannot even find in brackish water.
In oceans, they are found in all depths from
intertidal to the abyssal depths.
Usually found in the shallow water near the
shore- Adults are bottom dwellers
Sea stars - Carnivorous/predators (many);
Filter feeders
1. Feather stars -
sessile
Filter feeders
2. Sea lilies – sessile
Filter feeders
Brittle stars – free living Commensal with sponges
Sea daisy
Sea urchins - particle feeders
Feed mainly on algae or detritus
Sand dollars – bottom dwelling
• Particle feeders; eat tiny particles of food that
float in the water
Sea Biscuits- bottom dwelling
Sea cucumber
Suspension or deposit feeder:
eat decaying matter that floats in the water or in the
sand
Typical larval stage
Bipinnaria larva
• Free swimming
• Bilateral symmetry
• Most adult echinoderms show radial symmetry--Pentamerous symmetry (5 or more radiating areas)
(The first defining character of echinoderms)
As radial symmetrical animals;
• They lack cephalization; No head; No brain
• Hence, do not have anterior, posterior ends.
• Show, oral and aboral axis.
Aboral
Oral
• They do not have a highly developed nervous system;
includes circumoral nerve ring and radial nerves.
• A nerve ring that surrounds the mouth, helps to coordinate
their movements and responses.
• Sensory cells on their arms gather information about their
surroundings, such as light and temperature---light-sensitive eye spots---
• Echinoderms do not show metamerism
• Fluids within the well developed coelome cavity carry
out the function of diffusing substances and gases.
• Complete digestive gut is very simple leading directly from
mouth to anus; divided into a pharynx, stomach, intestine
and anus or cloaca.
• Circulatory system consists of a central ring and five radial
vessels; there is no true heart, and the blood lacks any
respiratory pigment.
• Gaseous exchange occurs by dermal branches or papulae in
seastars, peristominal gills in sea urchins, genitial bursae in
brittle stars and cloacal trees in holothurians.
• Exchange of gases also takes place through tube feet.
• Echinoderms lack specialized excretory (waste disposal)
organs and so nitrogenous waste, chiefly in the form of
ammonia diffuses out through the respiratory surfaces.
• Echinoderms possess a well developed internal skeleton
made up of dermal calcareous plates (CaCO3) (known as
ossicles)
(The second defining character of echinoderms)
Ossicles
• Ossicles are bound together with connective tissue.
• Spines and tubercles project out from these ossicles and
responsible for spiny body surface.
Dermal branchia
Muscles
Epidermis
Dermis
Body surface is marked by five symmetrically radiating areas (ambulacra)
and five alternating inter-radii (inter-ambulacra)
• Echinoderms possess a unique water vascular system or
ambulacral (appendages such as "tube feet") system. (The
third defining character of echinoderms)
• This is a network of fluid-filled canals that function in gas
exchange, and in locomotion.
• This system may have allowed echinoderms to feed without
the pharyngeal slits found in other deuterostomes.
• Water Vascular System is derived from a pair of coelomic
compartments; the hydrocele and axocele.
• The system comprises a ring canal, the hydrocele, that
encircles the gut and radial canals (ambulacrals) along the
body or arms.
• Water vascular system links to the outside seawater
through a sieve plate called madreporite.
• Madreporite leads to a vertical stone canal (made up of
spicules /plates of CaCO3 with hard and then to the ring
canal
• Five radial canals radiate symmetrically from the ring canal.
• Pairs of bulb-shaped ampullae connect to these radial
canals laterally.
• Each ampulla serves a single tube foot (Podia).
• Tube feet are tentacle like tubular extensions of the water
vascular system that project out from the epidermis and
penetrate ossicles.
• Both tube feet and ampullae are supported by ossicles
(ambulacral ossicles).
• Skeleton of particular region of tube feet known as
Ambulacral zone/Ambulacral grooves.
Polian vesicles also filter the sea water before enter into main body cavity
• Tube foot cannot extend itself as it does not have circular
muscles.
• Fluid which is pumped into tube foot by contraction of
ampulla able to extend hydraulically.
• The tube foot retract when the longitudinal muscles of the
tube foot contract.
Tube feet or podia, are sucker-like appendages that
the echinoderm can use:
• to move,
• grip the substrate,
• Gas exchange
• or manipulate objects
Classification
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Class Asteroidea
Class Ophiuroidea Asteriod-ophiuroid-echinoid body plan
Class Echinoidea
Class Concentricycloidea
Class Crinoidea - Crinoid body plan
Class Holothuroidea - Holothuroid body plan
Echinodermate body is constructed according to one of the
three above body plans.
Class Asteroidea =star like
(sea stars/ starfishes)
• About 2,000 species.
• They usually have five arms (rays)
hence, giving pentamerous symmetry.
Sea stars
• Sea stars are brightly coloured.
• They live on the rocky sea bottom; shore lines and among
coral reefs.
Arms are stout and short; merged with a central disc.
Ambulacral grooves are open on oral side of each arm.
Tube feet extend from each groove.
Tube feet project between ambulacral ridge.
• Move very slowly along the sea bed using hundreds thousands of tiny tube feet.
• Other than spines and tube feet
• Asteroids do possess;
• 1. noncalcified outfoldings of the outer body wall-papullae--serve as respiratory function
• (found protrud between ossicles and connects
directly to coelomic cavity)
• 2. Pedicellaria – ends of 2-3 ossicles that move
together with muscles, to remove unwanted
organisms and debris come in contact with surface of
the animal.
• Mouth leads to two part stomach; lower cardiac stomach
(large) and upper pyloric stomach (small).
• Pyloric stomach connects with digestive caeca located in
each arm.
• Short intestine leads from the stomach and connects to
anus on aboral side (upper side)
Brachiolaria larva of asteroid
• Bipinnaria
Brachiolaria
Subclass Ophiuroidea – Brittle stars
2000 species
• Brittle stars are found mainy on coral reefs (range from
shallow sea to abyssal depth).
• They have very long flexible arms (5) arising radially
from the central disc (help rapid movement).
• Arms can break off (and regenerated later).
• These broken arms will continue to wiggle and move to
distract a predator and allow the organism to escape.
A central, armored, disk-shaped body is clearly demarcated from the
arms.
• Ambulacral grooves are closed; tube feet are highly
reduced.
• Lacks anus and intestine; Mouth serve as the anus
Madreporite located on the oral side.
• Respiration by specialized pouches (bursae) located on
oral side near the bases of arms
These arms break of extremely easyly, hence the name "Brittlestars". The segments of the arms
are each covered by 1 - 3 plates (their number and shape help with the identification). The sides
of the arms are often spiny.
Ophiopluteus larva of ophiuroid
Class Crinoidea
• - Sea lilies and Feather stars
• These are elegant echinoderms with small, soft bodies that
are surrounded by 5 branched elongate and upwardly
raised arms, that each comprise of a central axis and then
numerous side branches(pinnules).
• They are characterized by being attached on to a substrate
for a long period in their life.
• Many are shallow water; some are in deep sea
• Body wall is leathery containing calcareous plates only on
aboral lower surface.
Class Crinoidea - Sea lilies, Feather stars
The oral surface of a feather star.
Food particals are then captured by the arms and are passed along
ciliated grooves back to the mouth (situated on the upper surface)
• Crinoids are pentamerous.
• A cuplike body bears five branched arms.
• Upper surface bears both mouth and anus, but do not
have a madreporite.
• Arms contain extensions of :
coelomic,
nervous,
water vascular, and
reproductive systems.
• Lateral pinnules are arranged on each arm.
• Pinnule bears an ambulacral groove bordered by fingerlike
tube feet used in suspension feeding and respiration .
Basket star
They poses a ring of claw-like segmented limbs (the cirri) beneath
the body with which they grip to the substratum, but they have
the abillity to crawl or even swim with the use of their arms.
An arm showing the lateral pinnulae
• A long stalk with cirri is attached on to the aboral side of
the body of sessile forms. (Sea lilies)
• Stalk is made up of plates appears as jointed stalk.
Doliolaria larva of crinoid
Class Holothuroidea - Sea cucumbers
• 900 species
• Body is greatly elongated in the oral-aboral axis.
• They have no arms.
• Ossicles are reduced hence, body is soft and muscular.
• Body wall is leathery with tiny ossicles embedded in it
forming spines.
• The mouth surrounds 8 - 30 retractable branched tentacles
(modified tube feet).
• Five double rows of tube feet run along the body; used for
crawling along the shallow sea bed or anchoring to a rock.
• Respiration & excretion by pumping sea water in & out of a
respiratory tree; tube feet and skin.
• Because of elongated body form of sea cucumbers, they lie
on one side.
• Tube feet are well developed only in the ambulacra applied
to the substratum.
• This provides the animal secondary bilateral symmetry.
Auricularia larva of Holothuroid
Class Concentricycloidea – Sea daisies
2 species
Tiny animals less than 1 cm diameter (disc shape)
Sea daisies were first discovered in sunken wood off the coast of New Zealand in 1983.
• Live 1 kilometre below the surface of the sea (abyssal
depth).
• The upper surface of the body is made up of numerous
perforated scales and is fringed with spines.
• They have no mouth, gut or anus; No arms
• It is assumed they get their food by absorbing nutrients
from decomposing logs of wood.
• Water vascular system (WVS) includes 2 concentric ring
canals; Tube feet arise from outer ring canal
Class Echinoidea - Sea urchins and sand dollars
950 species
• Sea urchins and sand dollars have large solid plates
around their soft bodies.
• They have movable spines; for locomotion.
• They do not have arms.
• Sea urchins have long, blunt spines serve as a defense
against predators.
• Sand dollars are flat and will spend most of their time
submerged under the sand in shallow waters.
• Most of these creatures have a globular body that is encased
in a hard calcium carbonate shell (the test). The test is formed
by the fusion of the spines in the skin. The mouth occurs
centrally on the underside, with the anus ussually on the
upper side. Through tiny pores in the test, tube feet protrude
(five double rows of tube feet run from the apex down the
sides of the test) and serve for movement for the sea urchin.
Long protective spines project from the test.
• Sand dollars have a series of pores arranged like a
flower on the dorsal body side
Internal structure of the sea urchin
Water vascular system
Portion of test
• Aristotle’s lantern -Complex chewing apparatus
Echinopleuteus larva of echinoids