To speed this lab, I would have 3 pairs working on sea urchins, 3 pairs working on
lancelots and 2-3 pairs working on tunicates. Then pairs should switch, so no one has to
wait. If you have a very cooperative specimen, please share your observations and
photographs. There should be enough specimens for most pairs do observe sand dollars
at the end or while waiting for another specimen.
Strongylocentrotus is a taxon of common regular urchins often used in
invertebrate zoology courses to exemplify echinoid anatomy. This exercise is written
specifically for the green sea urchin, Strongylocentrotus drobachiensis, but could also be
used for any other species of Strongylocentrotus, as well as Arbacia, Lytechinus, or
Echinus.
You should try to identify as many structures as you can in the sea urchin and then
compare it to the sand dollar you studied last week.
Behavior:
Obtain a living sea urchin and place in a small dish under the dissecting scope.
Focus on the tube feet and see if you can observe the cycle of expansion and contraction
of feet that enable the animal to move.
The function of the tube feet are obvious, but watch also the pedicellaria in action. Place
some dirt or dust near these jaws. They are actually defensive structures that keep small
organisms from settling on the surface. See section 5 for the observations you should
record in your journal.
1. Try to film tube feet in motion. Is there a rhythm to the motion and can you time
regular waver of contraction and relaxation?
Try to feed your specimen some seaweed. The specimen may be too traumatized to feed.
However if it does feed, observe the working of Aristotle’s lantern. This jaw like
structure can grind material before it enters the digestive tract. It is most often used to
scape algae and other vegetation off of rocks.
The diagram below shows the internal structure of a lantern, which of course will not be
completely visible unless you dissect the animal.
2. If your specimen feeds try to film the opening and closing of the jaws of
Aristotle’s lantern.
External Anatomy
Examine the living urchin. The lower, oral pole, is flattened whereas the
opposite, amoral pole, is more rounded. The animal is pentamerously radially
symmetrical.
The surface of the body is firm and rigid due to the underlying test of fused
calcareous ossicles. The test is hollow and most of the animal's soft parts are inside it.
The test is an endoskeleton, however, and is located in the connective tissue dermis of the
body wall and covered by a thin, inconspicuous, ciliated epidermis. Because the
epidermis is not readily apparent, the test appears to be external.
3. Spines
3a. If your specimen is living, touch one area of its epidermis with a needle
and watch the response of the nearby spines.
(Description):
Most of the surface bears articulated, movable spines, which are also part of the
connective tissue skeleton and are also covered by a thin epidermis. The spines of S.
drobachiensis have longitudinal ridges and grooves and are pointed at the apex, wider at
the base. Strongylocentrotus drobachiensis has large primary and smaller secondary
spines, of similar shape. (Some urchins have spines of different shapes and in some the
spines are all approximately the same size. Strongylocentrotus purpuratus has some small
cylindrical spines in addition to the larger conical ones. Arbacia punctulata has primary
but no secondary spines.)
The basal end of each spine bears a socket that articulates with a ball, or tubercle,
on the test. A ball and socket joint permits motion in a wide range of directions. An
outer ring of muscles and an inner ring of collagen fibers extend from the spine to the
test. Selective contraction of muscles in the outer sheath move the spine in any desired
direction atop its tubercle. The collagen fibers of the inner sheath can be locked so that
the spine is fixed rigidly in position. So effective is this locking mechanism that the
spines cannot be moved without breaking them. )
3b. Describe the different types of spines found on your specimen.
4. Tube Feet
4a. Remove a suckered tube foot and place it on a slide in a drop of bleach. Set it
aside until the soft tissue has been oxidized and bubbles cease to form and then affix
a coverslip. Examine the ossicles with the compound microscope and consider their
arrangement in an intact podium. Note that the flat ossicles are perforated by tiny
pores. The pores impede the propagation of cracks in the ossicle and are typical of
many echinoderm ossicles
(Description):
The 10 meridional rows of long slender tube feet, or podia, extend between the oral and
aboral poles. These rows are said to be meridional since they run from pole to pole as do
meridians, or lines of longitude, on a globe. The tube feet of regular urchins are normally
much longer than the spines but those of stressed or preserved animals are contracted and
shorter.
The 10 rows of tube feet are arranged into five ambulacra consisting of two rows
of tube feet each. Zones without tube feet, known as interambulacra, of which there are
five also, separate the ambulacra.
Most tube feet end in wide suckers used to hold the animal firmly to hard
substrata. The tube feet are used for locomotion and respiration, and some urchins, such
as Lytechinus variegatus, use them to hold bits of shell or vegetation above the body,
presumably for camouflage or protection from UV radiation in shallow water. The
behavior is known to be a response to high light intensities. The tube feet of the aboral
hemisphere of Arbacia are mostly suckerless and have a sensory role.
The sucker is reinforced with tiny flat ossicles that help it resist deformation
under stress. Deformation of the sucker, if it were allowed to occur, would result in loss
of suction. In addition, slender curved ossicles, resembling the spicules of sponges, are
present in the tube foot to support its walls. )
4b. Describe the different types of tube feet you see in your specimen.
.
5. Numerous pedicellariae (singular = pedicellaria) of several types are present on the
body surface and some can be seen in the vicinity of the mouth. Echinoid pedicellariae
have three tiny jaws at the end of a pedicle. Pedicellariae have a skeleton consisting of
three ossicles in the jaws and a long slender ossicle in the pedicle.
5a. Describe the different types of pedicellariae found on your specimen.
5b. Try to film the movement of a pedicellariae. It will take a while to observe the
jaws closing and opening, although pedicellariae waving should be most apparent.
5c. Remove a pedicellaria and make a wet mount. Examine the preparation with
the compound microscope and find the calcareous jaws and the calcareous rod in
the pedicle.
6. Photograph the dorsal and ventral surface and compare the structures found
there. As usual, a text description is furnished to aid your identification and relate
structure to function. Try to find the anus and madreporite on one surface and the
Aristotle’s lantern on the other.
Description of the aboral pole where the madreporite, anus and gonophore is found:
The periproct is a small region at the aboral pole surrounding the anus. The
periproct is much smaller than the peristome and, since it is surrounded by the spines of
the aboral hemisphere, is harder to see. The periproctal aperture through the test is
covered by the periproctal membrane, which in Strongylocentrotus is underlain by
numerous tiny calcareous ossicles. In Arbacia it is covered by four large, conspicuous
ossicles, the anal plates. The ossicles are not fused together, however, and the membrane
is flexible. The periproctal aperture cannot be seen in intact specimens. Use a strong jet
of water from a pipet or squirt bottle to wash the mucus from the aboral pole.
The periproct is at the exact center of the aboral surface but the anus is a little off
center, near one side of the periproct. The anus is a large opening on a slight elevation
and is surrounded by an irregular array of small spines. The elevation bears numerous
tiny, spiny knobs.
The periproct is surrounded by a ring of five large ossicles, the genital plates, and
five smaller ocular plates that are part of the rigid test and are fused to it. These cannot
usually be seen in whole specimens, as they are covered by the epidermis and spines.
One of the genital plates is modified to form a madreporite and this one is visible
externally. The large, triangular madreporite lies to one side of the periproct on an
interambulacral axis. Its surface bears obvious perforations and is covered by a ciliated
epithelium. The madreporite is penetrated by numerous ciliated channels continuous
with the stone canal and axial canal of the water vascular system in the interior.
The five genital plates, including the madreporite, each bear an opening, the
gonopore, through which a gonoduct opens. Although it may not be visible, the
gonopore of the madreporite can be found by probing the apex of the madreporite with a
microneedle. The needle will slip into the pore and pass completely through the test.
The gonopores are interambulacral.
Sand dollars: (Echinoidea) Do only after you have finished examining
the sea urchin and chordates available.
Clypeasteroids are flattened urchins known as sand dollars and sea biscuits
and adapted for living and moving infaunally in soft sediments. The test is
flattened to varying degrees on the oral-aboral axis and a secondary bilateral
symmetry is imposed on the underlying radial symmetry. Jaws and a
simplified Aristotle’s lantern are present. Respiratory tube feet are arranged
in petalloids.
.
Symmetry
Sand dollars are strongly depressed, or flattened, in a plane perpendicular
to the oral-aboral axis and are more or less disk-shaped The oral surface is
flattened or slightly concave and is the lower surface. The aboral surface is
convex and is upper.
Tubercles
Study the oral and aboral surfaces with high power of the dissecting
microscope. Most of the surface is covered with tubercles located in
shallow pits. In life, each tubercle articulates with a movable spine whose
motion is controlled by a ring of muscles running from the test to the base of
the spine .
6: Compare the size of the tubercles (and by inference the size of their
spines) on the aboral surface, oral surface, and margins of the lunules.
Where are the spines the largest?
Aboral Surface
Figure 1. Aboral surface of the keyhole urchin, Mellita
Petalloids
In irregular urchins some of the tube feet of the aboral surface are
respiratory whereas those of the oral surface are used for feeding and/or
locomotion. The aboral surface bears five conspicuous ambulacra, called
petalloids, in reference to their resemblance to the petals of a flower . The
areas between the petalloids are interambulacra. One petalloid points
anteriorly (12:00), two of them point antero-laterally (10:00 and 2:00), and
two postero-laterally (7:00 and 5:00). Under magnification you can see that
each petalloid consists of two rows of paired pores which accommodate the
ducts of respiratory tube feet as they pass through the test . Each tube foot
has a pair of pores and the two pores of each pair are connected by a
groove.
7: One of the five petalloids of the aboral surface of Mellita. Take a
photograph of a petalloid in your specimen .
(for your info) The tube feet associated with the petalloids are respiratory
and are not typical in shape or function. They are specialized gills with
enhanced surface area rather than being ordinary locomotory tube feet.
They do not have suckers, are not tubular, and are not used for locomotion.
Their shape facilitates an efficient countercurrent gas exchange mechanism
between seawater and the fluid of the water vascular system. Respiratory
tube feet are long, low, flat vesicles that lie in the groove extending from one
pore to the other. They are not slender tubes extending far away from the
body surface as are the more typical tube feet of regular urchins. Their long
axes are parallel, perpendicular to the ambulacral axis and to the surface of
the test. Their ampullae are similarly shaped but on the inside of the test .
The respiratory current generated by epidermal cilia flows over the surface
of the tube foot from the midline of the petalloid toward its side. The ciliated
peritoneum of the water vascular system inside the tube foot moves a current
in the opposite direction, from the side toward the midline. Oxygen,
following its gradient, diffuses into the tube foot from the seawater along the
entire area of contact between tube foot and seawater. Oxygenated water in
the tube foot then flows through the test via a median podial duct into the
ampulla. Once inside the ampulla, oxygen is transferred to the fluid of the
perivisceral coelom by another countercurrent mechanism, this one between
perivisceral coelom and water vascular system. The fluid in the ampulla,
now depleted of oxygen flows back through the test through the second of the
podial pores, this one lateral to the petalloid. Of the two podial pores flow is
from tube foot to ampulla in the medial pore and ampulla to tube foot in the
lateral pore.
8: Take a photograph of the different spines, tube feet, etc. in a lunule.
The test is perforated by five (six in some species, none in others) slots, or
lunules that pass entirely through it. In Mellita quinquiesperforata an
ambulacral lunule is situated near the periphery of the disk in each of the
five ambulacra except the anterior one . A fifth lunule, the anal lunule, is
closer to the center and is located in the posterior interambulacrum. In
juvenile dollars the lunules begin as notches in the disk margin which
deepen and then close as the animal grows. These are food gathering
devices. Although the mechanism are not known, somehow the appendages
bring water and food or just food from the aboral or upper surface to the
lower surface.
Aboral Ambulacral Center
Look at the center of the aboral ambulacral center with the high power of
your dissecting microscope . It is slightly anterior to the center of the aboral
surface.
The madreporite is a star-shaped area at the ambulacral center. Its surface
is perforated with tiny madreporic pores. These pores are lined with a
ciliated epithelium and connect with the axial canal and stone canal of the
water vascular system.
Nine pores penetrate the test around the periphery of the madreporite A
gonopore is situated at the apex of four of the five points of the
madreporite. The gonopores are interambulacral. The posterior
interambulacrum lacks a gonopore. (Encope has five gonopores, one in each
interambulacrum). The remaining five openings around the madreporite are
ambulacral and are tentacular pores for the exit of the terminal portion of
the radial water canal, which is called a tentacle.
Oral Surface
Examine the oral surface with your unaided eye . The lunules are, of
course, evident on this side also since they pass completely through the
body.
The oral surface of Mellita.
Peristome
The center of the oral surface is the relatively small peristome. The
peristomial aperture, houses the mouth at its center, a circular opening
located a little anterior to the center of the oral surface. It is covered by the
peristomial membrane in living dollars. The peristome coincides with the
oral ambulacral center from which radiate five ambulacra, known as
phyllodes.
The five narrow, incised food grooves, or ambulacral furrows, arise singly
from the margin of the peristome but each quickly branches to form two
food grooves that run beside the ambulacral axes, on either side of the
lunules. (The food grooves of Echinarachnius are long and straight and do
not branch until they near the periphery of the disk.)
Periproct
The periproct, which in life surrounds the anus, is a small opening in the
posterior interambulacrum between the peristome and the anal lunule . It is
near the mouth on the oral surface. (The periproct of Echinarachnius is
located at the center of the posterior margin of the disk.)
Podial Pores
With high power of the dissecting microscope, search the oral surface for
the unpaired pores of suckered tube feet. These are conventional, albeit
very small, tube feet with suckered tips. Each is served by a single podial
duct from its ampulla. The tube feet emerge from the test via tiny pores.
The pores are small but can be seen if you adjust the angle of the incident
light. The suckered podia are most abundant on the oral surface in the food
grooves. They are organized in leaf-shaped ambulacra known as phyllodes .
Aristotle's Lantern
The lantern of clypeasteroids, which are deposit feeders, is much simpler
than that of regular urchins, which are browsers, and essentially consists of
five jaws holding five teeth . Each jaw consists of a pyramid and a
compass.
Extra points for a great class photo of the madroporite or peristome or
food groove or periproct Aristole’s Lantern. You can only get credit
for one of these photo graps.. First to call it with the instructor is the
group that gets the points for that particular structure.