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Advanced Biology
Chapter 40 Echinoderms and
Invertebrate Chordates
Brittle Star
Focus Concept - Evolution
• Although Echinoderms are radially
symmetrical as adults, they are bilaterally
symmetrical as larvae suggesting that they
evolved from bilaterally symmetrical
ancestors
• Because both echinoderms and chordates
are deuterostomes, it is likely that they
have a common ancestor.
Chapter 40 Echinoderms and
Invertebrate Chordates
• 40-1 Echinoderms
• 40-2 Invertebrate Chordates
Characteristics of Phylum
Echinodermata
• Group of invertebrates that includes sea
stars, sand dollars, sea urchins, and sea
cucumbers
• Inhabit many marine environments from
shallow coastal waters to deep ocean
trenches
• Vary in diameter from 1cm – 1m
• Often brilliantly colored
• Adults have radial symmetry (like
cnidarians and ctenophores)
• Larvae have bilateral symmetry (unlike
cnidarians and ctenophores) which
indicates that echinoderms evolved from
bilaterally symmetrical ancestors
• No head or any other sign of cephalization
This one has 6 arms instead
of the normal 5
Echinoderm larva
• Fossil Record of Echinoderms
– Dates back to Cambrian period more than
500 million years ago
– Early echinoderms were sessile. Radial
symmetry is an adaptation to a sessile
existence
– Later evolved ability to move from place to
place. Most slow crawling on ocean floor. (80
species sessile)
• Deuterostomes (different from all other
invertebrates studied so far)
– Coelomates whose embryos have radial
cleavage. Anus forms near the blastopore and
mesoderm arises from outpockets of the
endoderm
– More closely related to chordates than to
other invertebrates
• Characteristics NOT shared by any other phylum
– Pentaradial symmetry (most): body parts extend from
center along 5 spokes
– Endoskeleton composed of calcium carbonate plates
known as ossicles. May have spines or spicules that
protrude through the skin. Echinoderm= “spiny skin”
– Water Vascular System: network of water-filled canals
used in movement
– Tube Feet: small, movable extensions of the watervascular system which aid on movement, feeding,
respiration, and excretion
Sea Urchin
Classification
Feather Star
• 7,000 species in 6 classes
– Class Crinoidea
– Class Ophiuroidea
– Class Echinoidea
– Class Holothuroidea
– Class Concentricycloidea
– Class Asteroidea
Sea Cucumber
Brittle Star
Class Crinoidea (Crinoids)
• Sea lilies, feather stars
• Crinoids = “lily-like”
• Sea lilies: most closely resemble fossils of
ancestral echinoderms from Cambrian
period. Sessile as adults. Attach to rocks
or sea bottom by long stalk
• Feather stars: can swim or crawl as adults,
may stay in one place for long periods.
Feather Star
Feather Star
Fossil Sea Lily
Sea Lilies
• Both types have 5 arms that extend from body
and branch to form many more arms (up to 200)
• Tube feet at the end of each arm filter small
organisms from the water. Also serve as
respiratory surface for O2/CO2 exchange with
the water.
• Cilia on arms transport trapped food to the
crinoids mouth at the base of the arms
• Mouth faces up (in most echinoderms the mouth
faces toward sea bottom)
A fossil of an ancient sealily, which probably lived 90
million years ago.
Class Ophiuroidea
• 2000 species of basket stars and brittle
stars
• Ophiuroidea = “snake tail”
• Long narrow arms – move more quickly
• Flexible arms branch repeatedly to form
numerous coils that look like tentacles
• Brittle stars: parts of arms break off easily
and can regenerate
Basket Star
Brittle Star
• Live on ocean bottom - beneath stones,
crevices, holes in coral reefs
• So numerous – cover sea floor in some
locations
• Some feed by raking in food with arms or
gather it from ocean bottom with tube feet
or with numerous mucous strands located
between spines
Class Echinoidea
• 900 species of sea urchins and sand
dollars
• Echinoidea = “spine-like”
• Test: compact, rigid endoskeleton that
encloses the internal organs of an
echinoderm
• Spherical sea urchin: well adapted to life
of hard sea bottoms.
– Tube feet for locomotion.
– Feed by scraping algae from hard surface
with 5 teeth surrounding mouth
– Aristotle’s lantern: complex jaw-like
mechanism of teeth and muscles
– Spines protrude from the test. Short and flat
or long and thin or wedge shaped depending
on the species. Some barbed others hollow
with venom (dangerous to predators as well
as swimmers)
Sea Urchin
Pencil
Sea
Urchin
• Sand dollars
– Live along sea coasts, sandy areas.
– Flat round shape adapted for shallow
burrowing
– Short spines used in locomotion, burrowing,
and cleaning body surface
– Use tube feet to capture food that settles on
or passes over their body
Sand dollar
Sand dollar
Mouthparts
Class Holothuroidea
•
•
•
•
Sea cucumbers
Holothuroidea = “water polyp”
Armless echinoderms
Live on sea bottom. Crawl or burrow into
soft sediment
• Ossicles that make up their endoskeleton
are very small and not connected. Body
soft
Sea Cucumber
Orange Sea Cucumber
• Modified tube feet form fringe of tentacles
around mouth. When extended –
resembles polyp form of some cnidarians
• Uses tentacles to sweep up sediment and
water. Stuff tentacles into mouth and
cleans food off them.
When threatened many species squirt out
noxious and sticky latex-like threads which
harden on contact with sea water, and
entangle or irritate the potential predator.
These are extremely difficult to remove.
If this fails, they will eject their entire
digestive system (gut) in an attempt to
scare the enemy and hence escape. The
gut can then be regenerated.
Some individuals may harbour
commensals inside their gut or
respiratory system. Two
prominent ones are the eel-like
pearl fish (Carapus) and the pea
crab (Pinnotheres).
Pentacta (length 6-8cm) is common on
sandy/muddy areas. Phyllophorus (up to 10crn)
(both F. Cucumariidae), which prefers muddier
substrates, has been reported to be used to
make a virtual "panacea" called "Air Gamat".
The animals are slit, their body fluids collected
and then left to stand for several weeks before
sale.
Most famous of all is H. scabra, the Beche-deMer or Trepang (F. Holothuridae) (length 1520cm), a much sought after delicacy. Degutted,
cleaned and dried, they fetch good market
prices. They prefer sandy areas.
The reef cucumbers (mostly F. Holothuridae)
grow to larger sizes (length up to 43cm). Their
colours vary between brown and black, and are
often seen in large numbers in the lower littoral
zone, usually, among Sargassum covered
rocks. Some of the reef species have been
reported to contain a poison called holothurin
and should not be eaten
Class Concentricycloidea
• Sea daisies (two species discovered in
1986 New Zealand)
• Deep waters
• Flat, disk-shaped. Less than 1cm in
diameter
• Tube feet around edge of disk (instead of
along radial lines)
Class Asteroidea
•
•
•
•
•
Asteroidea = “star-like”
Sea stars (starfish)
Live in coastal waters all over the world
Variety of colors and shapes
Economically important: prey on oysters,
clams, other organisms that humans use
as food
Bat Star
Short-spined Star
Ochre star (Pisaster
ochraceus) eating a chiton
Sunflower star (Pycnopodia
helianthoides)
Structure and Function of
Echinoderms
Acanthaster planci, or the crown-ofthorns starfish
External Structure
• Flattened body composed of several arms
extending from a central region (pentaradial
symmetry). Typically 5 arms, some as many as
24
• 2 rows of tube feet run along the underside of
each arm
• Oral surface: the side of the body where the
mouth is located (sea stars – underside)
• Aboral surface: side of the body that is opposite
the mouth
• Body usually covered with short spines.
Rough texture
• Pedicellariae: numerous tiny pincers
surrounding each spine. Help keep the
body surface free of foreign objects
including algae and small animals. *Not
found in other echinoderm classes
Water-Vascular System
• A network of water-filled canals that are
connected to the tube feet.
• Water Pathway:
– Madreporite: water enters system through small
pores. Sieve-like plate on the aboral surface
– Stone Canal: tube that connects the madreporite to
ring canal
– Ring Canal: tube that encircles the mouth
– Radial Canal: tube extends from ring canal to end of
each arm. Carries water to hundreds of hollow tube
feet
– Tube feet: valves prevent backflow into radial
canals from tube feet
• Ampulla: bulb-like sac on upper end of each tube
foot. Ampulla(latin)=“flask”. Surrounded by
muscles
• Sea stars use water pressure and muscle
contraction to extend and withdraw tube feet
• Many species have small muscles that raise the
center of tube foot’s disk-like end creating suction
cup for climbing slippery rocks and capturing prey.
Tube feet
Feeding
• Most are carnivorous: feed on mollusks, worms,
other slow-moving animals
• When captures bivalve (ex. Clam)
– Attaches tube feet to both halves of shell and exerts
steady pull.
– Clam’s muscles tire and shell opens slightly.
– Sea star inserts cardiac stomach and digests clam’s
soft tissues while still in shell.
– Then withdrawals stomach containing partially
digested food back into body to complete the
digestive process.
Sea star eating mussels
Digestion
• Mouth
• Esophagus
• Cardiac Stomach: turns inside out through
mouth when it feeds. Breaks down food with
enzymes
• Pyloric Stomach: Connects to a pair of digestive
glands in each arm. Breaks down food with help
of enzymes. Nutrients absorbed into coelom
through walls of digestive glands
• Anus: undigested material expelled. On aboral
surface
Other Body Systems
• No circulatory, excretory, or respiratory
systems
– Fluid in coelom bathes the organs and
distributes nutrients and O2
– Gas exchange and waste excretion take place
by diffusion through thin walls of tube feet and
through skin gills, which are hollow tubes that
project from the coelom lining to the exterior
Primitive Nervous System
• No head, no brain
• Nerve ring circles the mouth
• Radial nerves run from nerve ring down length
of each arm
• Coordinate movements of the tube feet. If radial
nerve cut, tube feet in that arm lose
coordination. If nerve ring cut, all arms lose
coordination
• Also nerve net near body surface that controls
the movements of spines, pedicellariae and skin
gills
• Eyespot on end of each arm responds to
light
• Several tentacles on end of each arm
respond to touch. Tube feet also
responsive to touch
• Touch-sensitive and chemical sensitive
cells scattered over surface of sea star’s
body
Reproduction and Development
• Most species have separate sexes (same as most
echinoderms)
• Each arm contains a pair of ovaries or testes. Females
produce 200 million eggs/year
• Fertilization occurs externally – eggs and sperm are
shed into water
• Bipinnaria: bilaterally symmetrical free-swimming larvae
that develops from the fertilized egg
• After 2 months, larva settles to bottom – metamorphosis
begins and it develops into pentaradially symmetrical
adult
Regeneration
• Can regenerate arms from central region of their body
even of all arms are lost
• Process very slow – takes as long as 1 yr.
• Use regenerative ability as defensive mechanism –
automatically shedding arm at base when arm captured
by predator
• Some can even regenerate a complete new individual
from a detached arm if it contains a portion of the central
region
• Certain species reproduce asexually by splitting body
through central region. Two parts then regenerate
missing structures.
Linckia regenerating five rays from a single surviving
ray. Specimens like this are often called "comets."
40-2 Invertebrate Chordates
40-2 Invertebrate Chordates
• Phylum Chordata includes vertebrates
(animals with a backbone) and 2 groups of
invertebrates (animals that lack a
backbone)
• Early development of chordates is similar
to development of echinoderms which
suggests that they descended from a
common ancestor.
Characteristics of Phylum Chordata
• Notochord: stiff, but flexible rod of cells that runs
the length of the body near the dorsal surface
– Stiffness provides resistance against which the body
muscles can exert force when they contract
– Flexibility allows body to bend from side to side and
up and down
– Some chordates retain notochord throughout life
– [In most vertebrate, notochord present in embryos but
becomes greatly reduced when vertebral column or
backbone develops. In adult mammals, the notochord
persists only as small patches of tissue between the
bones of the vertebral column
• Dorsal Nerve Cord: (unlike ventral nerve
cord of other invertebrates)
– Hollow tube with anterior end that enlarges
during development to form the brain
– Posterior end forms spinal cord
– Brain receives info from a variety of complex
sensory organs concentrated at anterior end
• Pharyngeal Pouches
– Out-pockets in the pharynx (portion of
digestive tract between the mouth and
esophagus)
– In aquatic chordates: pharyngeal pouches
became perforated by slits and evolved first
into filter-feeding structures and later into gill
chambers
– Terrestrial chordates: pouches evolved into a
variety of structures including jaws, inner ear,
tonsils
Link to facial development
• Postanal Tail
– Muscles in tail can cause it to bend
– Provides propulsion in many aquatic
chordates. (Invertebrates in other phyla lack
this form of propulsion because anus is
located at end of body)
Cat and human embryos in the tailbud stage. A cat embryo is shown on
left, a human embryo right. Note the post-anal tail in both, positioned at the
lower left below the head of each. The human embryo is about 32 days old.
Evolution and Classification
• Chordates are deuterostomes (like
echinoderms) – provides evidence that
they likely evolved from a common
ancestor
• Phylum Chordata divided into 3 subphyla
– Vertebrata (vertebrates) 95% of all chordate
species (Ch 41-45)
– Cephalochordata
– Urochorata
• Both subphylum live only in the ocean. Closest
living relatives to the early animals from which all
chordates evolved
Subphylum Cephalochordata
• 2 dozen species of blade-shaped animals known
as lancets
• Look much like idealized chordate in Figure 4010p788
• Retain notochord, dorsal nerve cord, pharyngeal
pouches, and postanal tail throughout life
• Live in warm, shallow water. Use muscular tail to
wriggle backward into sand. Only anterior end
protrudes from sand
Amphioxus
• Use cilia to draw water into pharynx
through mouth. Food particles in water are
trapped as it passes through numerous
slits in the pharynx. Food enters intestine
to be digested. Water leaves body through
opening called the atriopore
• Can swim weakly. Powered by
coordinated contraction of muscles that
run length of body. Muscles arranged as a
series of repeating segments.
– Body segmentation is another common
feature of chordates. Annelids and arthropods
are also segmented although it probably
evolved independently from chordates.
Subphylum Urochordata
• 2,000 species commonly called tunicates
– Body covered by a tough covering or “tunic”
– Also called sea squirts – squirt out a stream of
water when touched
• As adults, most are sessile, barrel-shaped
animals. Live on sea bottom
• May be solitary or colonial
Belize Painted Tunicate
Tunicate – Sea squirt
• Adapted for filter-feeding. Beating cilia
draw water in through incurrent siphon,
passes through slits in pharynx, exits
through excurrent siphon
– Food filtered by pharynx moves into stomach.
Undigested material leaves via anus which
empties into excurrent siphon
• Hermaphrodites: sperm and eggs released
through excurrent siphon into water where
external fertilization occurs
• Larval tunicates posses all 4 chordate
characteristics but lose most during
metamorphosis
• Adults bear little resemblance to idealized
chordate. Do have pouch-like pharynx with
slits, but no notochord, dorsal nerve cord
or postanal tail.