Invertebrate Anatomy OnLine
Clypeaster rosaceus ©
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises, a glossary, and chapters on supplies and laboratory techniques are
also available at this site. Terminology
and phylogeny used in these exercises correspond to usage in the Invertebrate
Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated
figure callouts refer to figures in the textbook. Callouts
that are not hyphenated refer to figures embedded in the exercise. The glossary
includes terms from this textbook as well as the laboratory exercises.
Eleutherozoa, Cryptosyringida, Echinozoa, Echinoidea C,
Clypeasteridae F (Fig
9-26, 27-12, 28-62)
are secondarily radially symmetric deuterostomes whose ancestors were
bilaterally symmetric. The
adult radial symmetry is pentamerous with body parts occurring in fives or
multiples thereof. Echinoderms
have strong affinities with the ancestral trimeric deuterostomes especially in
the tripartite organization of the coelomic cavities. Echinoderm
larvae have the coelom divided into three regions, as is typical of the early
coelomates, and these regions have important adult derivatives. All
echinoderms are marine and benthic. About 6000 Recent species are known but the
fossil record includes 13,000 extinct species.
important echinoderm apomorphy is the water vascular system that in most groups
functions in support of locomotory tube feet but is also important in gas
exchange, excretion, and feeding. The
body wall includes a thick connective tissue dermis in which calcareous ossicles
(little bones) are almost always an important component. These
ossicles make up an endoskeleton which assumes different forms in different
most echinoderms calcareous spines of various sizes and shapes arise from the
dermis and extend from the body surface and are alluded to by the name
echinoderm (= spiny skin). The
connective tissue is mutable and its consistency is under nervous control.
in echinoderms is accomplished by simple diffusion of metabolic wastes (ammonia)
across thin permeable regions of the body wall. A
variety of gas exchange structures, including the tube feet, is found in various
hemal system is present but its role in transport is still poorly understood and
the chief transport system is the circulating fluid of the various coelomic
hemal system may be through transport system that delivers nutrients from the
gut to these compartments for local distribution. The
nervous system consists of two central intraepidermal nerve rings from which
arise radial nerves to the periphery. Echinoderms
are gonochoric and fertilization is usually external.
are mobile echinoderms in which the oral surface is oriented against the
substratum. A madreporite and locomotory tube feet are present. Polian
vesicles and Tiedemann’s bodies may be present on the ring canal. Movable
spines are present. Eleutherozoa includes all Recent echinoderms except for its
sister taxon, Crinoidea.
includes Ophiuroidea, Echinoidea, Holothuroidea, all with closed ambulacra in
which the radial nerve is internalized and protected by ossicles (Fig 28-20).
sister taxon to Ophiuroidea, consists of the urchins and sea cucumbers. In
these echinoderms the oral surface and ambulacra have expanded aborally until
they enclose almost the entire body except for a small aboral periproctal region
with the anus. A
bony ring of ossicles surrounds the pharynx. The
hemal system is better developed than in other echinoderms. The
tube feet have ossicles.
includes about 950 extant species of sea urchins, sand dollars, sea biscuits,
heart urchins, and their relatives. The dermal ossicles are thin plates fused to
form a rigid, more or less spherical, endoskeletal test. Except
for a thin outer epidermis, all the soft anatomy is inside the test. The
larva is a bilaterally symmetrical echinopluteus. The
test is covered by an abundance of movable spines. Tube
feet are the major respiratory organs and the madreporite is aboral.
may be regular or irregular. Regular
urchins are the sea urchins,
with radial symmetry, globose nearly spherical bodies, and long spines. Most
are epifaunal. Irregular
urchinsare sand dollars, sea biscuits, and keyhole, heart, and cake
are usually infaunal in soft sediments and have a superficial bilateral symmetry
superimposed on their radial symmetry. The
body is usually flattened and the spines short.
species (regular urchins) possess a feeding apparatus known as Aristotle's
lantern, equipped with five strong teeth, used for scraping food from hard
lantern is reduced in infaunal species (irregular urchins) because most of them
are deposit feeders.
are flattened urchins known as sand dollars (Fig 28-42) and sea biscuits (Fig
28-41) 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. The
respiratory tube feet are arranged in petalloids.
depressed clypeasteroids are known as "sand dollars" because some of them have a
flattened disklike shape that resembles a silver dollar (Fig 28-42). Like
other irregular urchins most are infaunal deposit feeders with numerous short
spines that are used to burrow through soft sediments. As
an adaptation to facilitate movement through sediment, irregular urchins tend to
be elongate and slightly streamlined along an axis perpendicular to the
oral-aboral axis. This
imparts a secondary bilateral symmetry on the typical echinoderm radial symmetry
(which is why they are said to be “irregular”). Notice
that this is not the same as the bilateral symmetry of holothuroids, in which
the anteroposterior axis is the
aboral ambulacral center of clypeasteroids is at the approximate center of the
aboral surface and the oral ambulacral center is at the center of the oral
anus, however, does not coincide with the aboral ambulacral center (it does in
regular urchins) but occupies a position on the posterior part of the oral surface
or low on the posterior margin of the aboral surface (Fig 28-42). The
evolutionary migration of the anus away from its ancestral position at the
aboral pole can be followed in fossil echinoids. In
contrast, the mouth remains associated with the oral ambulacral center, and for
good reason. The
small tube feet of the oral ambulacra make up a set of radiating conveyor belts
that transport food particles from the periphery to the central mouth (Fig
28-43). No lunules are
rosaceus, the sea biscuit, is a good species for the study of the test of
irregular urchins (Fig 28-41A). It
is not a good choice for the study of internal anatomy because of the difficulty
of opening the thick test. This Caribbean species lives in and near beds of
turtle grass on sand in shallow water.
body is very thick, unlike those of other sand dollars. The
densely matted roots of turtle grass make digging difficult for such a large,
bulky animal and it does not burrow. Instead,
it uses the short spines of its oral surface to pole its way across the surface
of the sand. Clypeaster covers
itself with grass blades, shell fragments, and other debris making it very
difficult to see in its natural habitat. This
camouflage is held in place by suckered tube feet on the aboral surface.
contrast, its congener, C.
subdepressus, does burrow through sediment and is much flatter, being
shaped like a conventional, but very large, sand dollar. It
lives in areas of open sand free of grass roots.
rosaceus is longer over the
aboral-oral axis and narrower on the left-right axis. Its test wall is much
thicker. Bilateral symmetry is more evident in C.
comparison,Clypeaster subdepressus is
quite thin, presumably for cutting through the sediment. The periproct is on the
posterior margin of the oral surface in C.
rosaceus but farther in on the
oral surface inC. subdepressus.
and dried tests of Clypeaster
rosaceus and, optionally, C.
subdepressus, should be
available in the laboratory for study. Devote
most of your attention to C.
rosaceus but compare it with C.
subdepreessus when appropriate.
exercise is limited to study of the test and does not consider other aspects of
sea biscuit anatomy. Examine a
dried and bleached test of
the sea biscuit, C. rosaceus using
the dissecting microscope when needed (Fig 1). Do not use a pen or pencil as a
life the test is covered by a thin integument and is covered by short, dark
brown spines (Fig 28-41A). An
unbleached test in which the spines are still intact may be on demonstration. Visit
Figure 1. Aboral view of Clypeaster
The test is
a calcareous endoskeleton composed
of fused dermal ossicles. It
is rigid and, in C. roseaceus,
very thick (Fig 1). The convex, upper surface is aboral and
the lower, strongly concave, surface is oral. The
test is elongated along an axis perpendicular to the oral-aboral axis and this
symmetry over the original radial
bluntly pointed end of the body leads the way when the animal moves and is
referred to as anterior. The
opposite end, which is flattened, or truncate, is posterior. The
plane of bilateral symmetry includes both the antero-posterior
axis and the oral-aboral
species is not nearly so flat as most clypeasteroids so the name "sand dollar"
is not descriptive. It
is usually called a sea biscuit.
aboral surface bears a conspicuous flower-like arrangement of five ambulacral
areas called petalloids (=
petalloid ambulacra, Fig 1, 2). The
term petalloid is an obvious allusion to the petal of a flower. Each
petalloid consists of two rows of paired podial pores to accommodate two rows of
respiratory tube feet, or podia. Respiratory
tube feet have paired pores to accommodate the two connections each tube foot
has with its ampulla. Respiratory
tube feet are restricted to the aboral surface.
Figure 2. One of the five petalloids of the
aboral surface of Clypeaster
margins of the petalloid are formed by the paired pores. The
two pores of
each pair are connected to each other by a shallow conjugation
pairs joined by a groove are said to be conjugate.
the margins of a petalloid with magnification and find the two pores of a pair
and the groove connecting them. Notice
that the groove is shallow and does not penetrate the test but the pores do.
the aboral surface with high power of the dissecting microscope. Most
of it is covered by regularly spaced tubercles,
each set in a small depression, or pit. In
life, the short spines of the animal articulate with these tubercles (Fig
tubercles function as the male half of a ball and socket joint that allows great
freedom of movement of the spines (Fig 28-32A). Muscles
that operate the spine originate on the test around the circumference of the
spine and insert on its base. The
test surface between the pitted tubercles is covered with smaller tubercles that
do not occupy pits.
addition to the suckerless respiratory tube feet of the petalloids, conventional
suckered tube feet are also present. Clypeaster
roseaceus uses them to hold
grass blades and shells to its body and to move food particles toward the mouth. With
about 35X magnification, search the aboral surface for openings of ordinary, or
suckered, tube feet. Hold
the surface perpendicular to your line of sight and look closely and with
carefully adjusted light in the interiors of the petalloids. Here
you may see numerous tiny pores, for the exit of suckered tube feet. They
are difficult to see if the light is not shining from the correct angle. The
suckered tube feet are most abundant in the interiors of the petalloids or, put
another way, in or near the ambulacra, which, of course, is where you would
expect tube feet to be.
five petalloids radiate outward from the aboral
pole at the center of the
aboral surface. This
center of radiation of the ambulacra is the aboral
ambulacral center. One
petalloid points anteriorly, toward 12 on an imaginary clock face, and the other
four point to 2, 4, 8, and 10 (Fig 1). The
areas between successive petalloid ambulacra are interambulacra. They
are not as wide as the petalloids.
inconspicuous star-shaped madreporite is
situated in the aboral ambulacral center at the aboral pole (Fig 1, 2). If
you examine its surface with high power of the dissecting microscope, you will
see that it is perforated by tiny(!) madreporic
pores that, in life, open to
the axial canal and stone canal of the water vascular system. It
also bears tubercles for the articulation of spines.
the star-points of the madreporite, one in each interambulacrum, are five
conspicuous gonopores through
which the gonoducts from the gonads pass
this species the oral surface is strongly concave with the peristome at
the apex of the concavity near the center of the oral surface (Fig 4). The peristome is
the region surrounding the mouth (fig 3). In
life, the peristome is covered by the peristomial membrane with the mouth at its
center, as it is in regular urchins.
peristome coincides with the oral
ambulacral center although
the oral ambulacra are not as obvious as the aboral. Five
smooth ambulacral food
grooves radiate outward from
the ambulacral center to the periphery (Fig 3). In
position they coincide with the ambulacra, or petalloids, of the aboral surface
at 12, 2, 4, 8, and 10 o'clock. Clypeasteroids
are deposit feeders. Organic
particles are passed by small suckered tube feet along the food grooves to the
mouth (Fig 28-43).
The periproct is
near the posterior border of the oral surface (Fig 3, 4). In
life the anus is located at the center of the periproct which is covered by the
periproctal membrane. Note
that the periproct and anus are not located
at the aboral ambulacral center as they are in regular urchins.
the oral surface with high power of the dissecting microscope. Again
you will see pitted tubercles but
here they are larger and better developed than they are on the aboral surface. The
spines that articulate with these tubercles are the locomotory organs of the
biscuit and are stronger than those on the aboral surface. The
biscuit uses them to pole its way over the sediment surface.
high power of the dissecting microscope to look in the ambulacral areas for
podial pores. They
are tiny but abundant near the ambulacral food grooves but are scarce in the
middle of the interambulacral areas. The
oral tube feet have suckers and are used to move food to the mouth.
Figure 3. The oral surface of Clypeaster
a test that has been bisected along the median sagittal plane. This
is easily accomplished with a hacksaw but the instructor will probably provide
you with previously bisected biscuits. Notice
the thickness of the wall of the test and find the heavy, oblique pillars that
extend between the oral and aboral surfaces to strengthen the test (Fig 4).
pairs of erect auricles form
a circle, the perignathal ring, around the peristome surrounding the mouth (Fig
auricles insert into deep fossae on the surface of the jaws of Aristotle's
lantern and hold the jaws in place.
the pieces of Aristotle's
lantern if they are present. The
lantern, which is the jaw apparatus of urchins, is present among irregular
urchins only in clypeasteroids. In
these urchins it is large but is much simpler than the lantern of regular
urchins (Fig 28-38) and most of the ossicles present in regular urchins are
lantern and teeth of Clypeaster cannot
be extended out of the mouth as it is restrained by its articulation with the
lantern is made of five jaws,
each of which supports a hard, glossy, renewable tooth.
jaw is composed of two large, winglike, half
pyramids (= alveoli). The
other ossicles characteristic of the jaws of regular urchins are either absent
(e.g. compasses) or vestigial (e.g. rotules and epiphyses). The
half pyramids bear two flat surfaces and numerous thin lamellae that resemble
the gills of a mushroom. Each
pair of half pyramids forms a pyramid (=
jaw),which supports one tooth (Fig 28-38). The
two half pyramids of a pyramid articulate with each other by the smaller and
more irregular of their two flat surfaces.
Figure 4. Median “sagittal” section of Clypeaster
complete pyramid bears a conspicuous groove on its apex that encloses the tooth. Two
deep fossae on
the oral surface of the pyramid fit over the auricles of the perignathal ring. The
teeth are very hard orally but soft aborally. New
tooth material is added aborally as the hard oral end is worn away.
five pyramids are held together in a pentagonal star surrounding the peristome
by comminutor muscles (= interpyramid muscles) which run from the large flat
oval surface of one pyramid to the similar opposing surface of the adjacent
pyramid (Fig 28-38). Protractor
muscles extend from the aboral surface of the pyramids to the auricles of the
perignathal ring. In the clypeasteroids these muscles are not able to extend the
teeth out of the mouth as they do in the regular urchins. Retractor muscles
extend from the inner oral surface of the pyramid to the auricles.
one of the pyramids jaws)
in its proper position on an auricle. Then
imagine the other four jaws in their positions to get a mental image of the
LH . 1955. The
Invertebrates: Echinodermata, vol. IV. McGraw-Hill,
New York. 763pp.
PM, Grant RE . 1965. Echinoid
distribution and habits, Key Largo Coral reef Preserve, Florida. Smithson.
Misc. Coll. 149(6):1-68, 16 pls.
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
Teasing needle for use as a pointer
Cleaned test of Clypeaster
Cleaned test of C.
Bisected test of C.