Phylum Echinodermata

The echinoderms (“hedgehog skin”) are a very
unusual group that includes about 7000 living
species.

Members include: starfish, brittle stars, sea
urchins, sea cucumbers, and sea lilies or feather
stars.

They are deuterostomes (as are chordates), but
have secondarily evolved radial symmetry from
bilateral symmetry (they still have bilaterally
symmetrical larvae).
Characteristics of the
Echinodermata
 Exclusively
a marine group. They cannot
osmoregulate so rarely occur even in
brackish water.
 The
body is not segmented, but shows
pentaradial symmetry.
 There
is no head or brain and the nervous
system is relatively simple.
Characteristics of the
Echinodermata

They possess an endoskeleton of dermal
calcareous ossicles, which are connected
together by connective tissue.

Possess a unique water vascular system that
consists of a series of canals that extend from
the body surface as tube feet.

These tube feet are tentacle-like and enable the
animal to move. In some species movement of
the arms or spines contributes to locomotion too.
Classes of Echinoderms
 There
are a total of five classes of
echinoderms and about 7300 species.
 Class Asteroidea: sea stars or starfishes
 Class Ophiuroidea: Brittle stars
 Class Echinoidea: Sea Urchins, Sand
dollars
 Class Holothuroidea: Sea cucumbers
 Class Crinoidea: Sea Lilies and Feather
stars.
Class Asteroidea

Star fish are the most familiar of the
echinoderms and demonstrate the
characteristics of the group well.

Typically, they have 5 tapering arms, which
merge gradually with the central disc.

The mouth is found on the oral surface, the
opposite side of the animal being referred to as
the aboral surface.
Cushion seastar (Asteroidea)
Class Asteroidea

The aboral surface of starfish is usually rough to
the touch and has spines (although these may
be flattened so the animal appears smooth.).

Around the base of the spines are small
pincerlike structures called pedicellariae, which
are under muscular control and help to remove
debris and protect the animals papulae (skin
gills) projections of the coelomic cavity that
increase the surface area for gas exchange.
Figure 22.01
Figure 22.04f
Pedicellariae
Water vascular system
 Grooves
called ambulacral grooves
radiate out from the mouth on the oral
surface. Tube feet project from the
grooves.
 These
tube feet are connected to the
water vascular system, which is a unique
invention of the echinoderms.
Figure 22.02b
Water vascular system
 The
water vascular system is a
compartment of the coelom and is a
system of canals and tube feet.
 The
water vascular system uses hydraulic
pressure to extend, move and contract the
tube feet enabling the starfish to move and
feed.
Water vascular system
 The
water vascular system opens to the
outside via small pores in a structure
called the madreporite on the aboral
surface.
 The
madreporite connects to a ring canal
and from this ring canal, radial canals
extend down the abulacral groove of each
arm.
Figure 22.03b
Water vascular system
 From
each of the radial canals smaller
lateral canals branch off, which connect to
the tube feet.
 Each
tube foot is a hollow, muscle-lined
tube, which has a bulb at one end (on the
inside of the arm) and a sucker at the
other end which protrudes from the
animal.
Figure 22.03b
Figure 22.03c
Water vascular system
 The
tube feet use hydraulics to work.
 Each tube foot contains a lot of connective
tissue in its wall that maintains the tube at
a fairly constant diameter.
 Contraction
of muscles in the ampulla
forces fluid into the tube foot which
extends (a valve prevents fluid flowing
back into the lateral canal).
Water vascular system

To withdraw the foot, muscles in the tube foot
contract which pushes fluid back into the
ampulla.

The tube foot can bend to one side as a result of
the contraction of muscles on one or other side
of the tube foot.

In addition, small muscles at the end of the tube
foot can contract to raise the center of the tube
foot and create a suction cup.
Water vascular system

As a result of the combination of hydraulics and
muscular control the tube feet can be moved
with a high degree of control and can also exert
a strong pull.

This enables the starfish to move up hard
vertical surfaces and open up prey such as
bivalves.

On sediments suction doesn’t work well and
under these conditions the tube feet are used
more like legs.
Feeding and digestion
 The
mouth of a sea star opens into a twopart stomach in the central disk of the
animal.
 The
lower cardiac stomach can be everted
and this ability is important in sea stars
ability to consume certain prey.
Feeding and digestion
 The
cardiac stomach is connected to a
pyloric stomach above it which in turn
connects to digestive cecae in the arms
where most extracellular digestion takes
place.
 A short
intestine leads from the pyloric
stomach to the anus on the aboral side.
Figure 22.03a
Feeding and digestion
 Most
sea stars are quite unselective
carnivores and feed on molluscs,
crustaceans, annelids, other invertebrates
and other echinoderms.
 Some
sea stars are major predators of
economically important species of bivalve
such as mussels and oysters.
Feeding and digestion

When a sea star attacks a bivalve it wraps itself
around it and uses its tube feet to try to pull the
two shells apart.

The adductor muscles of the bivalve keep them
closed, but the sea star can maintain its pull for
long periods and eventually (after about half an
hour) the adductor muscles of the bivalve
become fatigued and the shells separate a little.
Feeding and digestion
 Once
a gap opens the sea star everts its
soft flexible stomach into the gap and
begins to secrete digestive enzymes that
slowly break down the soft tissues of the
bivalve
Figure 22.05a
Reproduction and development

Most sea stars have separate sexes and
fertilization is external.

Eggs are brooded by some species, but in most
cases the eggs are released to develop into
larvae.

Early development is typical of deuterostomes
and the free swimming bipinnaria larva is
bilaterally symetrical.
Figure 22.09a
Reproduction and development

Late in development, however, the larva follows
a path quite different from other deuterostomes.

The bipinnaria larva transforms into a
brachiolaria by growing three adhesive arms and
a sucker at its anterior end.

It then attaches to a substrate and undergoes
metamorphosis.
Figure 22.09b
Reproduction and development
 During
metamorphosis the larva is
radically altered.
 The
anterioposterior axis is lost and what
was the left side becomes the oral surface
and what was the right side becomes the
aboral surface.
Reproduction and development
 The
existing mouth and anus disappear
and new ones are produced in the
appropriate locations.
 Finally
short arms and podia develop and
the animal detaches from its stalk to
become a young starfish.
Figure 22.08
Regeneration
 Echinoderms
have the capacity to
regenerate lost parts. For example, a sea
star can regenerate a leg (or even all of its
legs) if it is lost).
 If
an arm that is removed contains at least
one fifth of the central disk a new
individual will develop from it.
Class Ophiuroidea

The Ophiuroidea includes the brittle stars and
basket stars.

This is the largest class of echinoderms with
more than 2000 living species and it probably is
also the most abundant group.

They are very common in all kinds of benthic
marine environments and abound on the
abyssal sea floor in many places.
Class Ophiuroidea
 Although
similar to sea stars in having five
arms, the brittle stars can be readily
distinguished by their proportionally much
longer and thinner arms and by the clear
separation between the arms and the
central disk.
Brittle star (Ophiuroidea)
Class Ophiuroidea
 Brittle
stars move in a different manner to
starfish. Their ambulacral grooves are
covered over with interlocking ossicles.
 They possess tube feet which play a role
in feeding, but don’t contribute to
movement.
 Movement instead occurs by movement of
the jointed arms.
Class Ophiuroidea

The mouth is surrounded by five moveable
plates that act as jaws.

Brittle stars lack intestines and an anus and
waste exits via the mouth.

Brittle stars feed on small particles that they
glean from the sea bottom or particles they sieve
out of the water with mucus strands attached to
spines on their arms
Figure 22.11
Class Echinoidea

The Echinoidea include the familiar sea urchins,
sand dollars, and heart urchins.

They have a compact body that is enclosed in a
calcareous shell (or test), which is formed from
dermal ossicles that have become a series of 10
double rows of close-fitting plates.

These plates also bear moveable, stiff, often
long spines.
Figure 22.15
Sea urchins
0126.jpg
Sand dollar
Figure 22.18a
Class Echinoidea
 The
sea urchins are considered to be
“regular” echinoids because they are
hemispherical and radially symmetrical.
 However,
the sand dollars and heart
urchins are considered “irregular” because
they have become secondarily bilaterally
symmetrical.
Class Echinoidea

In sand dollars the anus and mouth are located
on the oral side with the mouth near the middle,
but the anus shifted near the margin, so that an
anterioposterior axis is recognizable.

This is even more apparent in the heart urchins
where the mouth is moved towards the anterior
end and the anus (periproct) near the posterior
end.
Figure 22.17
Class Echinoidea

Echinoids are widely distributed in all seas. Sea
urchins are most typically found on rocky
substrates, but sand dollars and heart urchins
prefer sandy substrates.

Sea urchins feed by grazing on algae growing
on rocks and they do so using a complex
structure called an Aristotle’s lantern.
Aristotle’s Lantern
 The Aristotle’s
lantern is a chewing
mechanism that can be raised and
lowered by muscles inside the urchin.
 The
lantern consists of five plates
connected to each other with connective
tissue in a vase-like arrangement and
each plate has a tooth on the end.
Figure 22.19
Aristotle’s Lantern
 The
lantern can be lowered to scrape off
algae and then retracted inside using
paired muscles.
 The Aristotle’s
lantern connects directly to
the esophagus, which connects in turn to
the rest of the gut.
Class Holothuroidea
 The
Holothuroidea or sea cucumbers are
a quite odd group whose members at first
glance don’t seem that similar to the other
echinoderms.
 Sea
cucumbers are very elongated along
the oral-aboral axis and have an array of
oral tentacles (which are modified tube
feet) around the mouth.
Sea cucumber
Class Holothuroidea

Holothuroideans have a leathery body as the
ossicles are small.

Because they lie on one side, sea cucumbers
typically have well developed tube feet on only
the three ambulacra in contact with the
substrate.

The other two ambulacra have less well
developed tube feet, which are not used for
movement, but may play a sensory role.
Class Holothuroidea

Sea cucumbers are sluggish and don’t move
quickly.

They feed either on deposits, which they collect
as they crawl along the sediment or are filter
feeders that filter small particles from the water.

Deposit feeding sea cucumbers are a successful
group and make up as much as 90% of the
biomass on the surface of the deep-sea.
Class Holothuroidea
 An
unusual feature of sea cucumbers is
their respiratory tree, which is a many
branched internal respiratory structure that
connects to the cloaca.
 Water
is pumped in and out of the tree by
the muscular cloaca and the tree itself and
gas exchange takes place.
Figure 22.22
Class Holothuroidea

Attached to the respiratory tree are structures
called Cuverian tubules. These are used in
defense and are discharged out of the animal if
it is disturbed.

The tubules are long, sticky and sometimes toxic
and can entangle an attacker. Some species
also may discharge their digestive tract,
respiratory tree or gonads. These structures can
be regenerated.
Figure 22.23c
Class Crinoidea

Includes the sea lilies and feather stars, which
have an extensive fossil record and once were
much more abundant in the seas.

The crinoids have a very feathery appearance
that results from the branching of their five arms
to produce many more arms each of which has
many lateral pinnules.
Figure 22.25
Class Crinoidea
 Crionoids
occur most commonly in deep
water, but there are shallow water species
too.
 Not
surprisingly with their appearance they
filter feed and the food is carried down the
ambulacral grooves to the mouth by cilia
and tube feet.
Figure 22.24
Class Crinoidea
 Crinoids
have a similar water vascular
system to other echinoderms and have
only simple sense organs. They lack
spines, a madreporite and pedicellariae.
 Most
living species are not more than 12
inches long, but some fossil species had
stalks over 75 feet long.
Phylum Hemichordata

The hemichordates are deuterostome marine
animals that were previously classified as a
subphylum of the chordates because they
possess a dorsal nerve cord, gills slits and a
rudimentary notochord.

However, there is now consensus that the
hemichordate “notochord” is not homologous
with that of the chordates. Thus, they have been
classified in their own phylum.
Phylum Hemichordata
 The
hemichordates are wormlike
organisms that live on the bottom and are
usually found in shallow waters.
 There
are two classes: the Enteropneusta
(the acorn worms) and the Pterobranchia.
Acorn worms
 The
acorn worms are deposit or
suspension feeders that use their large
proboscis to trap food in mucus that is
then transported by cilia to the mouth.
Figure 22.29a
Saccoglossus an acorn worm (Hemichordata: class Enteropneusta)
Acorn worms
 The
larva of acorn worms is called a
tornaria and closely resembles the
bipinnaria larva of echinoderms, which
supports their classification as close
relatives of the echinoderms.
Figure 22.31
(Hemichordate)
(Echinoderm)
Pterobranchia

The pterobranchs are small colonial animals (17mm in length).

They live in tubes which they produce and can
freely move about in filter feeding using their
arms, which are covered in tentacles.

They lack a dorsal nerve cord, but one genus
has a pair of gill slits.
Figure 22.32
Cephalodiscus
a pterobranch hemichordate
Taxonomy of hemichordates
 Hemichordates
were once classified
among the chordates on the basis of their
gill slits and dorsal nerve cord.
 More
recently molecular sequence work
that has examined Hox genes and rRNA
places the henichordates in a clade with
the echinoderms. This is supported by the
similarities in larvae.
Taxonomy of hemichordates
 In
addition, research on an extinct goup
called the carpoids has found gill slits
which suggests that gill slits are ancestral
for all deuterostomes.
Figure 22.34