ECHINODERMS
ZLY 303:
Biology of Free-living
Non-Arthropod Invertebrates
Delivered by
SHITTU, Olalere
Dept. of Zoology,
University of Ilorin,
Nigeria
General Characteristics
• Adults exhibit pentamerous radial symmetry
• Radially symmetry is secondary; larvae are
bilaterally
symmetrical
and
undergo
metamorphosis to become radially symmetrical
adults.
• Poorly ganglionated;
structures
possess
• Body wall contains an
calcareous plates - ossicles
few
sensory
endoskeleton
of
General Characteristics
• Possess a network of canals throughout the body
- water vascular system.
• The canals are connected to extensions called
tube feet (=podia), located on the oral surface
• The water vascular system is important for
locomotion, feeding, and gas exchange.
• Sexes are separate; gametes shed into the water;
fertilization is external
General Body Wall
Morphology
Diversity
Class Asteroidea
• Sea stars, or starfish, vary in shape from nearly
circular, to pentagonal, to the familiar star-like
and flower-like forms
• Typically have 5 arms which merge with a
central disc with ambulacral area with holes for
the tube feet
• Mouth is located in the center of oral surface
which is directed downward
Diversity
Class Asteroidea
• The tip of each arm bears a tube foot that
functions as a sensory receptor for chemical and
vibratory stimuli, and some have a red pigment
spot that serves as a simple eye
• The outer surface consists of a latticework of
lime ossicles, or plates, between which project
thin-walled fingerlike extensions called papulae.
• The papulae and the tube feet are the principal
sites of respiratory exchange.
Diversity
Class Asteroidea
• In some groups of sea stars there are also body
wall projections called pedicellaria, equipped
with tiny pinchers that are operated by muscles
and are used to clean the body surface and
capture very small prey.
• They crawl about on rocks or muddy bottoms,
feeding on a variety of living and dead animals.
• Many feed largely on bivalve mollusks and are
notorious as destroyers of commercial oyster
beds.
Diversity
Class Asteroidea
• There are two or more gonads in each arm; at
spawning time these may nearly fill the arms.
The swimming larva settles and goes through a
sessile (attached) stage while changing to the
adult form.
Morphology
Water Vascular System
• On the aboral surface is the opening of the
water vascular system the madreporite (=sieve
plate)
• Water enters the madreporite and goes through
the stone canal to the ring canal and then passes
through a radial canal extending into each arm
• All along the length of these canals are lateral
canals that terminate in bulb-like structures
called ampullae equipped with tube feet
• Tube feet line the grooves on the oral surface ambulacral grooves
Water Vascular System in Sea Star
Cross Section of Sea Star showing the Arm
Groups of Starfish
• Brisingida - There are about 111 species of
Brisingida alive today. Members of this group
have between 6 and 20 long arms that they use
for suspension feeding.
• Forcipulatida - There are about 270 of
Forcipulatida alive today. Members of this group
have thick arms and body and includes many of
the starfish that inhabit temperate waters.
• Notomyotida - There are about 74 species of
Notomyotida alive today. Members of this group
live in deep sea habitats.
Groups of Starfish
• Paxillosida - There are about 370 species of
Paxillosida alive today. Members of this group
are among the more ancient of the living starfish
and lack more advanced features such as suckers
on their tube feet and an anus.
• Spinulosida - There are about 120 species of
Spinulosida alive today. Members of this group
have small spines on the upper surface of their
body.
Groups of Starfish
• Valvatida - There are about 700 species of
Valvatida alive today. Members of this group
include cusion stars and sea daisies.
• Velatida - There are about 140 species of
Velatida alive today. Members of this group are
deep sea species and live in cold water. They
have between five and fifteen arms.
Class Ophiuroidea
• The brittle stars, or serpent stars, are so called for
their long, slender, fragile arms, which are set off
sharply from the circular, pentagonal, or slightly
star-shaped body disk.
• The arms of brittle stars are flexible and appear
jointed because of the conspicuous plates of the
outer surface.
• They bear a row of spines along each edge. In
one group, the basket stars, they are repeatedly
branched, forming a large mass of tentacle-like
limbs.
Class Ophiuroidea
• Each arm contains a radial canal (or one of its
branches), but no body organs.
• They feed on detritus and small organisms. The
mouth leads to a large saclike stomach that fills
most of the body cavity.
• There is no intestine or anus, and solid waste is
extruded through the mouth.
• The stomach is folded into ten pouches, between
which lie ten respiratory sacs that open by slits
onto the oral surface.
Class Ophiuroidea
• The cells lining the sacs have flagellate, creating
a current of water moving in and out.
Respiratory exchange occurs chiefly through the
thin lining of the sacs.
Class Echinoidea
• They include sea urchins, heart urchins, and
sand dollars—are echinoderms without arms
and with a spiny shell, or test, formed of tightly
fused skeletal plates.
• The sea urchins (regular echinoids) are
hemispherical in shape, round on top and flat on
the lower surface. They have very long,
prominent spines and are often brightly colored.
• The test of a sea urchin is divided into ten parts
from pole to pole, like the sections of an orange.
Structure of Sea Urchins
Class Echinoidea
• Five of these are ambulacra, with openings for
tube feet; these alternate with wider sections,
called inter ambulacra that lack tube feet.
• Spines and pedicellaria are found over the entire
surface of the test.
• Urchins move by pushing against the substratum
with the spines and extending the tube feet in
the direction of movement.
Class Echinoidea
• The mouth, located in the center of the
undersurface, is surrounded by a thickened
region bearing five pairs of short, heavy tube feet
and sometimes five pairs of bushy gills.
• Within the mouth is an elaborate five-sided jaw
structure called Aristotle's lantern extrudes.
• It is able to grind up calcareous exoskeletons of
plants and animals.
• The anus is at the center of the aboral surface
and is surrounded by a thin-walled area without
skeletal plates.
Internal Structure of Sea Urchin
Irregular Echinoids
• Sand dollars and heart urchins have a dense
covering of short spines, and locomotion is
exclusively by movement of the spines.
• There are two groups of podia-bearing
ambulacra, one arranged in a petal-like pattern
on the upper surface and the other forming a
similar pattern on the lower surface.
• The upper tube feet function as respiratory
organs (there are no gills around the mouth),
and the lower ones are specialized for gathering
food particles.
Irregular Echinoids
• Sand dollars are extremely flattened and oval in
outline; the anus is on the oral surface.
• Heart urchins are somewhat flattened and are
heart-shaped; a deep ambulacral groove
running from top to bottom creates a secondary
bilateral symmetry.
• The anus is on the aboral surface, opposite the
groove.
Sea Urchins
• Sea Urchins
• Spherical body
• Ambulacral
plates
bearing tube feet that
radiate out toward the
aboral surface
• Use podia and spines
during locomotion
• The
spines
are
moveable
and
articulate with the
with the calcareous
ossicles
Sea Urchins
• complex chewing
apparatus called
Aristotle's lantern
• feed by
scraping
algae off
rocks
Class Holothuroidea
• The
sea
cucumbers
are
long-bodied
echinoderms with the mouth at or near one end
and the anus at or near the other.
• Because of their elongation along the oral-aboral
plane, they lie on their sides rather than on the
oral surface.
• In nearly all sea cucumbers the skeleton is
reduced to microscopic ossicles imbedded in the
leathery skin.
Class Holothuroidea
• Sea cucumbers have no arms, but tube feet
around the mouth have been modified to form a
circle of 10 to 30 tentacles of varying lengths
and shapes that function in gathering food
particles from the ocean bottom.
• The gut of the sea cucumber terminates in a
chamber called the cloaca that opens into the
anus. Two unique structures called respiratory
trees, found in most sea cucumbers, also
terminate in the cloaca.
Anatomy of Holothuroidea
Respiratory Trees of Holothuroidea
• These are systems of highly branched tubes, one
on either side of the body.
• The animal pumps water into the respiratory
trees by contracting the cloaca, and oxygen
diffuses through from the walls of the trees into
the fluid of the body cavity.
• The madreporite in most sea cucumbers opens
into the body cavity rather than to the outside
and receives its fluid from the cavity.
Respiratory Trees of Holothuroidea
• In a few sea cucumber species there is a large
mass of tubules at the base of the respiratory tree
that can be shot out of the anus if the animal is
irritated.
• The extruded tubules, which may engulf and
incapacitate an intruder, break off; they are then
regenerated by the sea cucumber.
• In other species the respiratory trees, gonads,
and part of the digestive tract are shot out
through the anus; this evisceration is followed by
regeneration of the lost organs.
Dermal Ossicles of Holothuroidea
• Dermal ossicles are usually microscopic bodies
buried in the leathery dermis.
• They can be extracted from tissues with
commercial bleach and are of important taxonomic
characteristics.
• They illustrate the meshwork Stereom structure at
some developmental stage in Echinoderms.
Class Crinoidea
• The sea lilies and feather stars are members of an
ancient group of stalked, sessile, detritus-feeding
echinoderms.
• Most of the sea lilies remain stalked throughout
life; their movements include bending the stalk
and the arms and crawling.
• Feather stars break off the stalk and become
free-living as adults.
• Crinoids, whether free or stalked, always have
the oral side upward, and the ring of arms about
the mouth gives them a flowerlike appearance.
Class Crinoidea
• They have at least 10 arms, but some sea lilies
have up to 40 and some feather stars up to 200
arms.
• The stalk and the arms have a jointed
appearance, and each arm has a row of
projections, the pinnules, on either side, giving a
feathery appearance.
• A ciliated ambulacral groove runs along each
arm and branches into the pinnules; the groove
contains feathery, tube feet arranged in triads.
Class Crinoidea
• The feet react to the presence of minute food
particles in the water by bending inward,
sweeping the particles into the groove, where
they are trapped in mucus and swept by the cilia
toward the mouth.
• Gametes develop in some of the pinnules, which
rupture at spawning time.
• The free-swimming larva eventually settles and
develops a stalk and a crown.
Anatomy of Crinoidea
Class Concentricycloidea
• The sea daisies, which were discovered in 1986,
have disk-shaped flat bodies and are less than
0.39 in. (1 cm) in diameter.
• There are two known species, viz: Xyloplax and
Dendrocystites.
• They have a water-vascular system, with tube
feet on the body surface around the edge of the
disk.
• They have no obvious arms or mouth, and
appear to absorb nutrients through the
membrane surrounding their bodies.