BIOL 202
LAB 13
Echinodermata & Hemichordata
Introduction
The Phylum Echinodermata includes some of the most strange and interesting groups of
marine organisms. As noted in the previous chapter, Echinoderms are deuterostomes and
have radial indeterminant cleavage. Animals in this group are either radially symmetrical
or biradially symmetrical and are unsegmented. These feature are found in groups that
we have already discussed. However, the Echinoderms have a number of characteristics
that are quite unusual and unique to this group.
The feature that gives the phylum its name is a dermal skeleton that is often spiney and
is composed of calcareous ossicles or spicules. This skeleton is covered by a layer of
epidermis, making this the first group we have studied to have an internal skeleton. The
organ systems of echinoderms are also quite different. The digestive system consists of a
mouth, found on the oral side; a short esophagus; either a one or two part stomach;
digestive glands; and an anus. Excretory organs are entirely absent in this group.
The most unique organ system is the water-vascular system, which is derived from the
coelom. This versatile system provides locomotion, excretion, respiration, and food
gathering functions. The entire system opens to the outside of the organism through a
small pore called the madreporite and projects from the body surface of the animal by
tentacle-like projections called podia or tube feet. By controlling the hydraulic pressure
in the water-vascular system, the animal can retract or project tube feet to aid in
locomotion. Respiration is accomplished via dermal branchia (skin gills), through the
tube feet, or by a respiratory tree in the Holothuroids.
Echinoderms have no heart and a much reduced blood-vascular system. This simple
hemal system is surrounded by portions of the coelom and body fluids are moved by cilia
rather than being pumped by a heart. These organisms have no head or brain. There are
some specialized sensory organs such as chemoreceptors and photoreceptors. The
nervous system is composed of a ring of nerves around the oral area, with radial nerves
projecting from the ring.
The reproductive system consists of large gonads with simple ducts. There is no
copulatory apparatus, although the sexes are usually separate. Fertilization is generally
external, but some species are known to brood their eggs. The larvae of echinoderms are
bilateral at early stages and later take on radial symmetry as their development proceeds.
Yet another unique feature of the Phylum is the ability of animals to regenerate lost body
parts. In the past, shellfish fisherman often dealt with starfish caught in their nets by
cutting them into pieces and throwing them back into the water, not realizing that instead
of killing one starfish, they had created several new individuals. Holothuroids (sea
cucumbers) are known to have the distasteful habit of eviscerating themselves when
threatened by predators. They can then regenerate a new digestive system within a short
period of time.
1
The Ecology of Echinoderms
Echinoderms are an entirely marine group. They cannot osmoregulate, and are almost
never found in brackish water. They can be found in almost all other marine habitats,
from intertidal areas to abyssal depths. Most of them are bottom dwellers, however,
Holothuroids and Ophiuroids (brittle stars) are quite capable of swimming.
Holothuroidea includes some very odd animals. They are elongated and their ossicles
are much reduced and buried in a leathery dermal layer. They generally crawl along the
bottom, but are capable of burrowing and swimming. Sea cucumbers gather food by
using their tentacles to collect suspended food particles from the surrounding water or
from the sea bottom.
Ophiuroidea (brittle stars) includes some of the most delicate creatures to be found.
This class contains the largest number of species of echinoderms, and they may be the
most abundant. In many abyssal areas they literally carpet the sea floor. Brittle stars
generally prefer the deeper ocean areas, and many species are actually negatively
phototropic. They use their arms to filter suspended particles or browse on the bottom.
Brittle stars are also capable of graceful swimming over short distances.
Crinoidea (sea lillies and feather stars) are sessile filter feeders that live attached to the
substrate. The Crinoids are a primitive group that was once quite abundant, with some
giant species attaining several meters in length. Although much less plentiful today,
crinoids can still be found in many of the world’s oceans. They also use their long
tentacles to gather suspended food particles.
Echinoidea is the class that includes sea urchins, sand dollars and heart urchins. Sea
urchins generally live on rocky or hard substrates, while sand dollars and heart urchins
burrow in the sand. Echinoids have an elaborate chewing system known as Aristotle’s
Lantern. This structure is composed of a complex set of muscles that control the
movements of five teeth that are used for grazing on the organic materials found on rocky
or hard substrates.
The most ecologically important and familiar group of Echinoderms are the Asteroidea
or starfish. Sea stars prey on molluscs, crustaceans and many other invertebrates. They
feed on oysters and clams by prying open the shell slightly with their arms and extruding
their stomach into the shell, where digestion takes place. In many marine ecosystems,
they are the top predators. Some species of starfish have also caused major damage to
coral reefs in many areas.
Human Relevance
The most important impact that Echinoderms have on humans is their role as a pest in the
oyster and clam industries. One starfish can consume up to a dozen oysters or clams in a
day, and have caused major losses in the shellfish industry. In an effort to control their
numbers, fisherman have resorted to netting them, and to distributing lime over the areas
in which they are found. The lime destroys the dermal branchia of the starfish, leaving
the oysters and clams unaffected.
Echinoderms have also been widely used in experimental embryology. The animals are
relatively easy to handle and maintain, while a few individuals can produce a huge
number of gametes.
2
3
Lab Exercise — Starfish Dissection
External Anatomy
Before beginning your dissection, you need to locate the external features on your
starfish. Notice the pentaraidal symmetry (five arms projecting from the center disc).
Does your specimen have five arms or is one of them missing or smaller than the rest?
Notice the spines covering the upper (aboral surface) of the animal. Between these
spines you should see some very thin-walled tissue. These are the dermal branchia. If
you can’t see them well, place your specimen under a dissecting microscope. While you
are observing the dermal branchia, see if you can locate the pedicellaria. They are tiny
pincer-like appendages located at the base of the spines. Pedicellaria function to help
protect the dermal branchia, prevent larval barnacles from settling, and may aid in food
gathering. Look at the tip of each arm. Can you see the eyespots. Once again the
dissecting microscope may be needed. Another feature on the aboral (away from the
mouth) surface of your starfish is the madreporite. It functions as the entrance to the
water-vascular system and is located at a point where two of the arms meet.
Turn your animal over. The first thing you should notice is the groove running down the
center of each arm. This is the ambulacral groove which is enclosed by a protective
layer of ossicles, the ambulacral ossicles. The tube feet are located in rows inside this
groove. In the center of the disc is the mouth. It may be protected by five moveable
spines. In some individuals, the stomach may be everted through the mouth. Check your
specimen to see if that is the case.
To begin the dissection, cut off the tip of one of the arms. Be sure that it is NOT one of
the rms bordering the madreporite. Next, cut toward the enter along either side of the
arm on the dorsal surface, and remove the dorsal part of the arm carefully. The first thing
that you will see is the pyloric cecum (digestive glands). This organ is quite extensive,
and takes up most of the space in all five arms. Carefully lift up the pyloric cecum, and,
if you have a mature specimen, you should be able to see the gonads. Look carefully,
they may not extend as far toward the tip of the arm as the pyloric cecum. Also notice
the long tube that extends down the center of the pyloric cecae. This is the pyloric duct
extending outward from the pyloric stomach in the central disc.
Lift both the pyloric cecum and gonads out of the way and lay them off to the side.
Below that you will see rows of tiny thin-walled, bulb-like structures. These are the
ampullae, a part of the water-vascular system. If you look closely, you will note that
they extend through the dermal ossicles and connect with the tube feet. The ampullae
have circular muscles, and when contracted, force water into the tube feet. When the
tube feet are contracted, with their longitudinal muscles, the water is forced back into the
ampullae.
Once you understand the structure of the arm of a starfish, the next step is to examine the
central disc. In order to do this, you will need to repeat the dissection procedure that you
performed, on two additional arms. These must be the other two arms that do not border
the madreporite. Try to leave the madreporite and the arms bordering it intact for you
later examination of the water vascular system. Once again, cut off the tip of the two
arms you wish to dissect. Then make cuts along the dorsolateral margin on either side up
to where they join the central disc. Very carefully lift up the loosened dorsal surface and
free any material that may be clinging to it. You must be extremely careful at this
4
juncture, as the pyloric stomach is a very thin-walled organ lying just beneath the surface.
Try to locate the pyloric ducts radiating out in five directions from the pyloric stomach.
Also located on top of the pyloric stomach is the rectal ceacum, which consists of two or
three lobes. This opens outside to the anus, which you cannot see. Beneath the pyloric
stomach is the large cardiac stomach. When the starfish feeds, it grasps its prey (usually
clams and oysters) and uses the suction of the tube feet to pry it open just a bit. It then
everts the cardiac stomach through the mouth and into the prey where it digests the soft
internal tissue.
The next part of the dissection will be the water vascular system. After you are confident
that you understand the digestive system, carefully remove the stomach from the central
disc. The opening of the water vascular system is the madreporite, which you have
already located. From the madreporite, a small curved white tube extends toward the
interior of the animal. This is the stone canal. This leads to the ring canal, which
encircles the central disc. Look very carefully and be gentle. These structures are quite
fragile, but they can be found. From this ring canal, five radial canals extend out into
the arms. The radial canals probably cannot be seen in this dissection, but may be seen in
the prepared cross-section slides. Very short lateral canals extend from the radial canals
to the ampullae, completing the water vascular system.
Your final task for this dissection is to take a look at the prepared slides. In the crosssection of the starfish arm, you should be able to locate the pyloric ceca, gonads (if
present), spines, dermal branchia, the ambulacral ossicles, ambulacral groove, radial
canal, lateral canals, radial nerve, blood vessel, ampullae, and the tube feet.
Slides: ZJ 1-2, ZJ 1-5, ZJ 1-41
5
6
Phylum Hemichordata (Amphioxus) Slide ZL 1-1
7
Echinoderm Classes
The class Asteroidea contains the sea stars. The surface with the madreporite plate is the
aboral surface; the surface with the mouth is the oral surface—it is normally oriented
downward. Sea stars have five arms with ambulacral grooves, containing tube feet,
running down the length of the arm. Sea stars are predators, and may feed on small
marine organisms, or larger bivalves by pulling the shell apart using the tube feet. When
feeding on large bivalves, when the gape of the bivalve is open slightly, the cardiac
stomach of the sea star everts into the bivalve, and partial digestion occurs, which
weakens the adductor muscles of the bivalve, allowing the starfish to open the shell to a
greater gape and completely ingest the bivalve. The pyloric stomach connects to ducts
that lead to pyloric cecae (two in each arm). A short intestine then leads to rectal cecae
and to an anus on the aboral surface of the central disk. Sea stars have dermal branchiae
for gas exchange, and many have pincer-like pedicellaria that clean the surface of the
body and are protective Most sea stars are dioecious with external fertilization. Their
reproductive processes and embryological development of the ciliated, free-swimming
larvae have been extensively studied. Sea stars may regenerate lost arms.
The class Ophiuroidea contains the brittle stars and basket stars--the arms of ophiuroids
are distinctly set off from the central disc. The arms may branch. Ophiuroids lack dermal
branchiae and pedicellaria; the tube feet lack suction discs and ampullae and are not used
for locomotion. Ophiuroids have closed ambulacral grooves; rather they use the skeletal
ossicles for locomotion. Movements of the arms are snake-like; hence the meaning of the
class name. The madreporite is on the oral surface. This is the most speciose group of
echinoderms. Ophiuroids are dioecious, and males are so small that females may carry
them around. The larvae, the ophiopleuteus, is planktonic. Ophiuroids can also replace
lost arms, and indeed may shed them easily (autonomy) as an escape reaction.
The class Echinoidea contains the sea urchins, sand dollars and heart urchins. The body
is globular and there are no arms in this group. Sea urchins have long tube feet and
pedicellaria; the pedicellaria in some species have venom. Sea urchins are adapted for
living on hard surfaces; sand dollars and heart urchins live in or on soft substrates (some
are burrowers). Sea urchins have a specialized chewing structure (Aristotle’s lantern) for
grazing on algae, but sand dollars and heart urchins use their tube feet to trap organic
particles. Echinoids are dioecious; they shed gametes into the water, so fertilization is
external. The larval form is a free-swimming pluteus, but it eventually settles to the ocean
bottom and metamorphoses into a sessile adult.
The class Holothuroidea contains the sea cucumbers. Sea cucumbers have no arms, and
are secondarily bilaterally symmetrical. They are elongated along the aboral/oral axis.
Sea cucumbers are soft bodied with only microscopic ossicles; the madreporite is
internal, and they lack spines or pedicellaria. They have tube feet typically on one side
(the side they lay on); other tube feet surround the mouth and function as tentacles. They
are deposit feeders or suspension feeders. The dioecious sea cucumbers typically
reproduce with external fertilization, but some brood their eggs. Sea cucumbers may also
reproduce asexually by transverse fission; they later regenerate lost parts.
8
The class Crinoidea contains about 630 species of sea lilies and feather stars. They are
the most primitive and unusual echinoderms—crinoids were abundant in the Paleozoic
and although there are extant forms, most biologists have not even seen a living
specimen. They have branched arms and use the tube feet for suspension feeding—
trapped food items are carried to the mouth via cilia in the ambulacral grooves. This may
be the ancestral form of feeding for the echinoderms. Sea lilies are attached by stalks;
feather stars can swim and crawl. Some crinoids are dioecious, but others are
monoecious. Some crinoids brood embryos on their arms. Crinoids can regenerate lost
parts.
9