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Chapter 29:
Echinoderms and Invertebrate Chordates
Section 1: Echinoderms
Echinoderms
• Includes starfish, sea urchins, sand dollars, etc.
• Belong to the phylum Echinodermata
– Stretches back to the beginning of the
Cambrian Period, more than 580 million
years ago
What Is an Echinoderm?
• Echinoderms are spiny skinned animals
• In addition to having a spiny skin, echinoderms are
characterized by five-part radial symmetry, an internal
skeleton, a water vascular system, and suction-cuplike
structures called tube feet
• The internal skeleton, or endoskeleton, is made up of
hardened plates of calcium carbonate, which are often
bumpy or spiny
• The water vascular system consists of an internal network
of fluid-filled canals connected to external appendages
called tube feet
– Water vascular system is involved in many essential life
functions in echinoderms
Form and Function in
Echinoderms
• Adult echinoderms have a body plan with five
parts organized symmetrically around a center
• Typically have neither an anterior nor a
posterior end and no brain
• The side where the mouth is located is called
the oral surface, and the opposite side is called
the aboral surface
Form and Function in
Echinoderms
• The water vascular system opens to the outside
through a sieve like structure called the
madreporite
• The entire water vascular system operates like
a series of living hydraulic pumps that can
propel water in or out of the tube feet
• The tube feet act like living suction cups
• All echinoderms “walk” with their tube feet,
and some use their tube feet for feeding
Feeding
• Carnivores
– Use their tube feet to pry open the shells of bivalve mollusks
such as clams and scallops
– Once opened, the carnivores flips its stomach out of its
mouth, pours out enzymes, and digests its prey in the prey’s
own shell
• Herbivores
– Scrape algae from rocks by using their five-part jaw
• Filter feeders
– Use tube feet on flexible arms to capture plankton that float
by on ocean currents
• Detritus feeders
– Move like a bulldozer across the ocean floor, taking in a
mixture of sand and detritus
Respiration
• Echinoderms need to exchange carbon
dioxide for oxygen
• In most species the thin walled tissue of
the tube feet forms the main respiratory
surface
• In some species small outgrowths called
skin gills also function in gas exchange
Internal Transport
• The functions of transporting oxygen, food,
and wastes are shared by different systems
in echinoderms
• The distribution of nutrients is performed
primarily by the digestive glands and the
fluid within the body cavity
Excretion
• Solid wastes are released through the anus
in the form of feces
• Excrete nitrogen-containing wastes
primarily in the form of ammonia
• Wastes seem to be excreted in many of the
same places around the body in which gas
exchange takes place – the tube feet and the
skin gills
Response
• Have primitive nervous systems
• Have a nerve ring that surrounds the mouth
and radial nerves that connect the ring with the
body sections
• Also have light sensitive cells to help them tell
whether it is night or day
• Many echinoderms hide under rocks and in
crevices by day, coming out to feed at night,
when most predators are asleep
Movement
• Most echinoderms use tube feet and thin
layers of muscle fibers attached to the plates
of the endoskeleton to move
• Mobility is determined by the structure of
the endoskeleton
– Some plates are fused
– Some have flexible joints
Reproduction
• Most are either male or female
• Some are hermaphrodites
• In starfish, the sperm and egg are produced in
testes or ovaries which fill the arms during the
reproductive season
• Shed their sperm and eggs into the water
• Larvae swim around for some time
• Eventually, they will swim to the ocean
bottom, where they mature and metamorphose
into adults that have radial symmetry
The Echinoderm Classes
• Almost 6000 species of living
echinoderms
• Found in almost every ocean in the world
• No echinoderms have ever entered fresh
water, and they cannot survive for long
on land
• Echinoderms are remarkably diverse in
appearance
Starfish
• Contains the common starfish, which are also
known as sea stars
• Occur in many colors
• Many species have more than 5 arms
• Starfish creep slowly along the ocean bottom
• Most are carnivores, preying upon the bivalves
they encounter as they move
• Some species are important predators in rocky
areas along the coast
Brittle Stars
• Live in tropical seas, especially on coral reefs
• Look much like a common starfish, but they have
longer, more flexible arms and are able to move
much more rapidly
• Brittle stars protect themselves by shedding one or
more of their arms when attacked
• The detached parts keep wriggling violently,
distracting predators, while the rest of the animal
escapes
• Are filter and detritus feeders that hide by day and
wander around in search of food at night
The brittle star gets its name
from the fact that it can shed
its arms when it is threatened.
This distracts predators so that
the brittle star can escape. In
time, it will regrow the missing
arm. Some starfish, such as the
sun star, have more than five
arms.
Sea Urchins and Sand Dollars
• Includes disk-shaped sand dollars, oval heart
urchins, and round sea urchins
• Most are grazers that eat large quantities of algae
• Others are detritus feeders
• Heart urchins and sand dollars live hidden in
burrows
• Most sea urchins wedge themselves in crevices
in rock during the day and only come out at
night
• Many sea urchins have long, sharp spines
The slate urchin has thick, strong spines
that were once harvested for use as
implements for writing on slateboards.
Sea urchins have a lanternlike set of
bony plates inside their body that power
their jaws. The sand dollar gets its
name from its flattened, coin-shaped
appearance.
Sea Cucumbers
• Look like warty moving pickles with a mouth
at one end and an anus at the other
• Most are detritus feeders
• Not numerous in shallow water
• Herds containing hundreds of thousands of
them often cover areas of the sea floor at great
depths
• A few species expel sticky substances that
attach to a predator
• The predator is immobilized as it is glued into
a helpless ball
Sea Lilies and Feather Stars
• These filter feeders, which have 50 or more
long, feathery arms, comprise the most ancient
class of echinoderms
• Not common today
• Sea lilies are sessile animals that are attached
to the ocean bottom by a long, stem like stalk
• Many feather stars live on coral reefs, where
they perch on top of rocks at night and use
their tube feet to catch floating plankton
How Echinoderms Fit into the
World
•
•
•
•
•
Numerous in most marine habitats
Control the populations of other animals
Control the distribution of algae
Considered delicacies by some people
Useful as research subjects and as
possible sources of medicine
Chapter 29:
Echinoderms and Invertebrate Chordates
Section 2:
Invertebrate Chordates
Invertebrate Chordates
• Phylum Chordata
– Fish, frogs, birds, snakes, dogs, cows, and
humans
• Most of the chordates are vertebrates,
which means they have backbones, so they
are placed in the subphylum Vertebrata
• The invertebrate chordates are divided into
two subphyla – tunicates and lancelets
What Is a Chordate?
• Belong to phylum Chordata
• Chordates are animals that are
characterized by a notochord, a hollow
dorsal nerve cord, and pharyngeal
(throat) slits
• All chordates display these three
characteristics at come stage in their life
st
1
Characteristic
• The notochord, is a long, flexible
supporting rod that runs through at least
part of the body, usually along the
dorsal surface just beneath the nerve
cord
• In most vertebrates, the notochord is
quickly replaced by the backbone
nd
2
Characteristic
• The hollow dorsal nerve cord, runs
along the dorsal surface just above the
notochord
• In most chordates, the front end of this
nerve cord develops into a large brain
• Nerves leave this cord at regular intervals
along the length of the animal and
connect to internal organs, muscles, and
sense organs
rd
3
Characteristic
• Pharyngeal slits, are paired structures in
the pharyngeal, or throat, region of the
body
• In aquatic chordates the pharyngeal slits
are gill slits that connect the pharyngeal
cavity with the outside
• Many invertebrates have gills of some
sort in various places, but only chordates
have pharyngeal gills
rd
3
Characteristic
• In terrestrial chordates that use lungs for
respiration, pharyngeal slits are present only for
a brief time during development
• These slits soon close up as the embryo develops
• In chordates such as humans, pouches form in
the pharyngeal region but never open up to form
slits
• For this reason, some scientists regard
pharyngeal pouches, not slits, as the “true”
chordate characteristic
Tunicates
• Small marine chordates that eat plankton they filter
from the water
• Get their name from a special body covering called
the tunic
• Only the tadpole-shaped larvae of tunicates have a
notochord and a dorsal nerve cord
• When tunicate larvae mature, they undergo
metamorphosis and become sessile adults that grow
into colonies attached to a solid surface
• Both larval tunicates and adults filter feed and breathe
at the same time through a pharyngeal basket pierced
by gill slits
Lancelets
• Small fishlike creatures that live in the sandy
bottom of shallow tropical oceans
• Have a definite head
• Have a mouth that opens into a long pharyngeal
region with up to 100 pairs of gill slits
• Feed by passing water through their pharynx,
where food particles are caught in a sticky mucus
• This mucus is swallowed into a digestive tract that
starts at one end of the pharynx and continues
straight through the animal to the anus, near the
tail
Lancelets
• Have a simple, primitive heart that pumps blood
through vessels in a closed circulatory system
• They show evidence of segmentation in the
arrangement of their nerves and muscles
• Muscles are organized into V-shaped units that are
paired on either side of the body
• Each muscle unit receives a branch from the main
nerve cord
• Lancelets have no jaw
• Their mouth is composed entirely of soft tissues
• Also lack appendages and can move only by
bending their bodies back and forth
How Invertebrate Chordates Fit
into the World
• It is important to remember that living
vertebrates did not evolve from living
lancelets or tunicates
• Both these subphyla have evolved over time
• However, similarities in structure and
embryological development indicate that
vertebrates and invertebrate chordates
evolved from common ancestors many
millions years ago
Homework
• Page 646 #1 – 3