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LABORATORY EXERCISE VII
PHYLUM ECHINODERMATA echinoderms
These spiny animals have a radial symmetry that is secondary, i.e., it is derived from a
bilateral ancestor. Echinoderm symmetry is also described as pentamerous—can you
determine what this means? The generally hard endoskeleton is due to plates, called
ossicles, which are joined together in various ways. Echinoderms are coelomate and possess
a unique body system called the water vascular system. This system is involved in
locomotion in many echinoderms.
CLASS ASTEROIDEA sea stars or starfish
Sea stars typically have five arms that grade into the central disk, but there is much variation,
as seen in the specimens on demonstration.
Asterias Starfish arm c.s. Slide (Box V slide tray 8; 4x)
Be able to recognize the ampullae,
podia and suckers of the tube feet,
which are part of the water vascular
system. Be able to distinguish them
from the coelom. Also be able to
find the pyloric ceca and the
radial canal.
Asterias Sea star specimen
The common starfish of the New
England coast may be used to
illustrate the general echinoderm
structure. Be able to label the tube
feet, madreporite, central disk
and the mouth.
Starfish diversity Both wet and dry preserved specimens
Be able to recognize the starfish
on demonstration as members of
the phylum Echinodermata, class Asteroidea.
Sea stars have remarkable powers of
Regeneration—with at least 1/5 of the
central disk present, a sea star will
regenerate. One of our specimens
shows the regenerative process.
CLASS OPHIUROIDEA serpent stars or brittle stars
The five arms of serpent stars are usually distinct from the central disk.
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Brittle stars specimens
Notice that each arm is distinct
from the central disk. Notice the
articulated plates of the skeletal
system of the arms that give them
great flexibility and mobility.
Basket starfish or Gorgon's head
These highly modified ophiuroids are
common in the cold ocean waters of
northern Maine and Canada and is also
found in the tropics. But they are not
common in temperate waters.
CLASS ECHINOIDEA
Echinoids are without arms and are spherical or flattened in form, but they still retain the
pentamerous symmetry characteristic of the phylum. Spines are very noticeable in most
echinoids and vary in shape, size, and number from species to species. The class includes
the spherical sea urchins and the flattened cake urchins, sea biscuits, and sand dollars.
Sea urchins
The numbers and shape of spines
differ greatly among sea urchins, as
demonstrated. Look for the tube feet.
Sea urchin skeleton
The bleached-out skeleton of sea
urchins, common in many "shell
shops," is made up of fused ossicles.
In living animals, spines are located
on the knobs, and tube feet project
through the holes. Using this skeleton,
can you discover evidence of pentamerous
symmetry?
Aristotle's lantern
The spherical sea urchins possess
a unique and intricate oral grinding
device known as Aristotle's lantern.
They use this structure to feed on
algae growing on rocks or coral.
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Cake urchin skeleton
The cake urchin or sea bisquit
comes from shallow waters in the
subtropical and tropical Atlantic.
Living specimens have spines similar
to sea urchins. Can you see the
pentamerous symmetry
Sand dollars (preserved and bleached skeleltons)
These sand dollars are common
in waters along coastal Carolina.
Actually, they can be found
in virtually any shallow water marine
habitat. Note the lunules—holes in
their skeletons. The function of these
structures is not known but may help
the animal maintain stability in currents.
CLASS HOLOTHUROIDEA sea cucumbers
Sea cucumbers are usually elongated and soft creatures. Ossicles are reduced to individual
microscopic plates imbedded in the skin; there are no spines. Feeding tentacles usually
surround the mouth, which are highly modified tube feet.
Sea Cucumbers
What evidences of pentamerous symmetry
do you see on these specimens?
Dissected specimen
This dissection is to allow you to
examine the respiratory trees, a
unique holothuroidean structure.
Also be able to locate the gut and
the gonads.
CLASS CRINOIDEA sea lilies and feather stars
Extant sea lilies are stalked and attached deep water forms. Feather stars are without stalks
and can swim and walk short distances.
Crinoid fossil
Notice the stalk and the arms of
this fossilized crinoid. Crinoids dominated
during the Paleozoic but today there are
only about 500 extant species.
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PHYLUM HEMICHORDATA acorn worms
Acorn worms are delicate, though usually large inhabitants of marine mud. They have
pharyngeal gill slits and a dorsal, occasionally hollow nerve chord (thought by most
scientists to be homologous to the chordate nerve chord). But they have no notochord
(although early researchers mistakenly thought they did, hence they were once classified as a
chordate). Thus we call them hemichordates. Hemi is Greek for half.
Acorn worms
Notice the collar, proboscis and trunk. These
marine worms usually live in muddy bottom sediments.