Echinoderms

Echinoderms
1
Phylum Echinodermata :
About 7,000 species
Strictly marine, mostly benthic.
Typical deuterostomes
Diversity :
 Echinodermata means “spiny skin”

Echinoderms usually inhabit shallow coastal waters and ocean trenches

organisms in this class include:
•
Sea stars
•
Brittle stars
•
Sand dollars
•
Sea cucumbers
Characteristics :
 change from a free-swimming bilaterally symmetrical larva to a bottom-dwelling
adult with radial symmetry.

Most have five radii or multiples which is known as pentaradial symmetry

they have an endoskeleton that is made up of calcium plates, may include
protruding spines
 Have small feet called tube feet that aid in movement, feeding, respiration, &
excretion.

Do not have circulatory, respiratory of excretory systems.

Have a nervous system but no head or brain.

There are two sexes and they can produce sexually and asexually.
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Evolution & Classification :
 Echinoderms are from the Cambrian period & date back to over 500 million years
ago

scientist believe that they evolved from bilaterally symmetrical ancestor.

The inferred ancestral larva is very similar to the modern Sea star larva.

Records show that conditions have changed which had caused them to evolve from
sessile organisms to free-living ones.
Taxonomists have divided 7,000 species of echinoderms into
five classes:
 Crinoidea

Asteroidea

Ophiuroidea
 Echinoidea

Holothuroidea
Crinoidea (“lilylike”) :
 They include:

Sea lilies

Feather stars

they have long stalks that attach to rocks or to the
ocean floor

feather stars eventually detach themselves

Sticky tube feet that are at the end of each arm
catch food and serve as a respiratory surface.
Crinoidea are sessile
Asteroidea (“star-like”)
 starfish or sea stars belong in this class

found all over coastal shores around the world

prey on oysters, clams, and other sea food that is used by people
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Ophiuroidea (“snakelike”)
 largest echinoderm class

includes basket stars & brittle stars

primarily reside under stones & in crevices and holes of coral reefs

have thin brittle arms that break off & regenerate themselves quickly

feed by raking food off the ocean floor with their arms and bottom of tube feet

also trap food with mucous strands between their spines.
Echinoidea (“hedgehoglike”)
 sand dollars & sea urchins

test: rigid endoskeleton that the internal organs are compacted in

Aristotle’s lantern: complex jaw-like mechanism that is used to grind their food

locomotion: tube feet
protection: barbs on their long spines that are sometimes venomous
Sand dollars :
 live along seacoasts & sandy areas

flat, round shape bodies; and adaptation for shallow burrowing

locomotion: short spines (also aid in burrowing & cleaning their bodies)

use tubes to filter food out of water
Holothuroidea :
 sea cucumbers belong in this class

bodies are soft
 feeding: tentacles around the mouth sweep up sediment from the water

protection: eject internal organs through the anus.

Lost parts are later regenerated.

Process called evisceration
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Structure & Function
Body Plan of the Sea Star :
 oral surface: mouth located on the underside of the body

aboral surface: top of the body

ossicles: sharp protective spines made of calcium plates, covered with thin
epidermal layer

pedicellariae: tiny forceps that protect and clean the body surface
a partially dissected view of the aboral (upper) surface of a of
a starfish
7 coelom
madreporite
stone canal
arms
central disc
Cardiac stomach
Pyloric stomach
dermal ossicles
pyloric ceca
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anus
ampullae
ambulacral ossicles
13 gonads
•
The image contains the principal internal organs and some elements of the watervascular system.
•
Asteroids normally have five rays, or arms, (1)
•
arranged around a central disc (2).
•
Most echinoderms have a complete digestive system with a mouth (found on the
oral surface of the starfish), a cardiac stomach (3) , a pyloric stomach (4),
5
•
pyloric ceca (5) that take up most of the space in the arms and an anus (6).
•
Although radially symmetrical as adults, echinoderms possess a well developed
coelom (7) lined with cilia that keep water moving through the organism.
•
Structures of the water vascular system that can be seen include the entrance to
the system called a madreporite (8) that leads to a vertical stone canal (9).
•
From there water enters a ring canal that surround the mouth, then into radial
canals in each arm that deliver water via lateral canals to the bulb-like structures
called ampullae (10).
•
It is the contractions of these ampullae that deliver fluid under pressure to extend
the tube feet so that they can make contact with the substrate or a food item.
•
The lateral canals are contained within bony elements called ambulacral ossicles
(11) that form an ambulacral ridge in each arm (seen as ambulacral grooves on the
oral surface of each arm)
•
. Starfish also have a well developed internal skeleton made up of bony elements
called dermal ossicles (12).
•
Also seen on the model are several of the gonads (13), which are occupy a
proximal position in each arm.
Water-Vascular System :
 hydrostatic pressure permits movement

Path of water in the Water-Vascular System
u
enters sieve plate
u
passes through stone canal
u
traces a path from the ring canal encircling mouth to 5
radial canals that extend to each arm
u
ampulla: bulblike sac that each foot connects to
u
feet contract, water enters and are able to suction onto
surface of slippery rocks
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Sea Star Anatomy
Feeding & Digestion :
 uses feet

eat mollusks, worms, and slow-moving animals

enzymes help digest food
Other Body Parts :
 fluid in coelom bathes organs & distributes nutrients & oxygen

skin gills: protect coelom lining; gases are exchanged

nerve ring: surrounds mouth & branches off into nerve cords in each arm.

Eyespots: on each arm that responds to light
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
tentacles: responds to touch
Reproduction :
 each arm produces sperm & egg

occurs externally

bipinnaria: free-swimming larva that a fertilized egg develops into

settles in the bottom and develops into an adult through metamorphosis

reproduce asexually by regenerating lost parts
Body wall :
 Epidermis covers entire body.
 Endoskeleton of ossicles with tubefeet and dermal branchia protruding through and
spines and pedicellaria on outside.
 Ossicles can be fused into a test (urchins and sand dollars).
 Ossicles spread apart in cucumbers.
 Ossicles intermediate and variable in seastars.
 Muscle fibers beneath ossicles.
 Tubercles and moveable spines on skeletal plates of echinoids.
 Small muscles attach spines to test.
 Pedicellaria in echinoids and asreroids.
 Respond to external stimuli independent of nervous system.
 Keep debris and larvae from settling, protection, hold on to material for
camouflage.
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Water vascular system :
 Fluid-filled canals for internal transport and locomotion.
 Fluid similar to sea water but has coelomcytes and organic molecules.
 Moved through system with cilia.
 Asteroidea:
 Madreporite on aboral surface. Grooved with ciliated epidermis. May allow
seawater into vascular system.
 Ampulla under madreporite connected to water vascular system and hemal
system.
 Stone canal connects ampulla to rest of system. Connects to ring canal.
 Ring canal leads to radial canals in each arm. Also has Polian vessicles (maintain
internal pressure) and Tiedemann’s bodies (produce coelomcytes).
 Radial canals :
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 lead to lateral canals which pass through pores in the skeletal plates and end in
tube feet.
 Each tube foot has an ampulla on top and a suckered muscular podium on bottom.
 Tube feet used for locomotion, prey capture, adherence to substratum.
 Terminal tubefeet are chemosensor .
Water vascular system :
 Tube feet move by combination of muscles and hydraulics.
 Valve at lateral canal that shuts and isolates the tubefoot.
 Ampulla contracts and pushes fluid into the tubefoot to extend it.
 Sucker pressed on substratum and sticks with adhesive secretions.
 Longitudinal muscles contract to raise middle of sucker to create a vacuum. Also
shortens podium, forcing water back into ampulla.
 For release, longitudinal muscles relax, ampulla contracts and water forced back into
podium. Suction released .
Water vascular system in Ophiuroids:
 Madreporite on oral surface.
 Tudefeet don’t have suckers.
 Flexible used for feeding.
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Water vascular system in Crinoids :
 Water vascular system entirely coelomic fluid.
 No madreporite, many stony canals.
 Radial canals extend up each Arm.
 Suckerless podia on branches called pinnules.
Water vascular system in Echinoids:
 Madreporite on special plate around aboral pole.
 Podia pass through holes in ambulacral plates
Water vascular system in Holothuroids:
 Madreporite internal and open to coelom.
 Three rows of tube feet (trivium) on “ventral” surface, two rows (bivium) on
“dorsal” surface.
Movement :




Ophiuroids : use flexible arms for crawling
Urchins : use tube feet and moveable spines.
Sand dollars :use spines to burrow in sand.
Cucumbers : crawl on podia of trivium or by muscular action of the body wall.
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Nervous system :
 Decentralized without cerebral ganglia.
 Relatively simple receptors: chemoreceptors, statocysts, touch.
 Some brittle stars have sclerites that act as tiny lenses across their dorsal surface
and work together as one giant lens
feed and digest :
Crinoids :
 Filter feed with oral side up and arms and pinnules outstretched.
 Food particles brought to mouth via cilia in ambulacral grooves.
 Mouth opens to short esophagus, to long intestine, to anus
13
Asteroids :
 Most are predators and scavengers.
 Eversible portion of stomach (cardiac stomach) extruded onto or into prey.
Asteroids :
Mouth ---> cardiac stomach ---> pyloric stomach ---> pyloric ducts ---> pyloric cecae --->
intestine ---> anus
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Ophiuroids :
 Predators, scavengers, filter feeders, deposit feeders.
 Food collected and passed along podia and spines to mouth.
 Digestive system reduced with no anus.
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Echinoids :
 Herbivores, suspension feeders, detritovores.
 Urchins have Aristotle’s lantern.
 Hard plates and muscles that control protraction of five teeth.
 Teeth scrape algae off rocks and take bites of macroalgae.

Can excavate holes in rocks.
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Echinoids :
Digestive mouth system ---> esophagus out of Aristotle’s lantern ---> long intestines --->
rectum ---> anus.
 Holothuroids : Suspension and deposit feeders.
 Extend mucus-covered buccal tentacles into water. Tentacles are pushed into
mouth one at a time.
 Mouth ---> esophagus ---> long intestines ---> rectum ---> anus.
 Cuverian tubules - blind sticky tubes at base of respiratory tree.
 Entangle predators.
 Evisceration.
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Circulation :
 Internal transport by coeloms, water vascular system, and hemal systems.
 Hemal system - array of canals and spaces enclosed within coelomic channels called
perihemal sinuses.

Parallels water vascular system.
 Probably helps distribute respiratory gases and nutrients.
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Echinoderms maintain homeostasis :
Gas exchange :
 Across podia and dermal gills (dermal branchia).
 Countercurrent exchange
 Ophiuroids :
 have ten invaginations in the body wall called bursae.

Water circulated by cilia.
Holothuroids :
 have respiratory trees.
 Water is actively pumped by muscular hind end.

Gases picked up by coelom and hemal system.
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Osmoregulation :
Osmoconformers.
 Waste is usually ammonia lost across podia and dermal branchia.
Sexual reproduction :
 Most gonochoristic.
 Gonads housed in genital sinuses.
 In classes with multiple gonads, each has own gonopore in an interambulacral
area.
 Free spawning with indirect development to brooding with direct development.
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Sexual reproduction
 Isolecithal egg with small amount of yolk.
Radial holoblastic cleavage ---> coeloblastula ---> coelogastrula by invagination --->
blastopore becomes anus ---> coelom formation by enterocoely ---> embryo becomes
bilaterally symmetrical and develops into a larva.
 Isolecithal egg with small amount of yolk.
Radial holoblastic cleavage ---> coeloblastula ---> coelogastrula by invagination --->
blastopore becomes anus ---> coelom formation by enterocoely ---> embryo becomes
bilaterally symmetrical and develops into a larva.
 Most capable of regenerating lost parts.
 Holothuroids regenerate intestines and respiratory trees.
 Asteroids and ophiuroids regenerate lost arms and suckers.
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How do Echinoderms reproduce and develop?
Sexual reproduction
Isolecithal egg with small amount of yolk.
Radial holoblastic cleavage ---> coeloblastula --->
coelogastrula by invagination ---> blastopore becomes
anus ---> coelom formation by enterocoely ---> embryo
becomes bilaterally symmetrical and develops into a larva.
Ophiopluteus of brittle
star
Echinopluteus of
urchin.
Aricularia of sea
cucumber
Sea Cucumber Anatomy
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