Invertebrate Anatomy OnLine
Copyright 2001 by
is one of many exercises available from Invertebrate
Anatomy OnLine , an
Internet laboratory manual for courses in Invertebrate Zoology. Additional
exercises can be accessed by clicking on the links on the left. A
glossary and chapters on supplies and laboratory techniques are also available. 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.
Medusozoa, Hydrozoa C,
Campanulariidae F (Fig
cnidarian body consists of a central blind sac, the coelenteron (=
gastrovascular cavity), enclosed by a body wall comprising two epithelia, the
outer epidermis and the inner gastrodermis (Fig 7-1, 7-2). A
gelatinous connective tissue layer, the mesoglea, lies between the two
epithelia. The mouth opens at one end of the coelenteron and marks the oral end. The
mouth is at the tip of a process, the manubrium that elevates it above the oral
surface. The opposite pole is the aboral end. The
imaginary line connecting the oral and aboral poles is the axis of symmetry
around which the radial symmetry of the body is organized. The
mouth is usually surrounded by one or more circles of tentacles.
defining cnidarian feature is, of course, their possession of stinging cells, or
cnidocytes (Fig 7-8). Characteristic
of the epidermis, they are also sometimes found in the gastrodermis. Cnidocytes
contain an explosive organelle, the cnida, which, upon proper stimulation,
inverts and ejects a slender, often barbed and toxic thread in the direction of
prey or predator (Fig 7-9). Three types of cnidae are found in cnidarians (Fig
(in nematocytes), spirocysts (in spirocytes), and ptychocysts (in ptychocytes). All
toxic cnidae are nematocysts whereas spirocysts are sticky, and the everted
tubules of ptychocysts are used for constructing feltlike tubes. Most
cnidae are nematocysts and these are present in all three higher cnidarian taxa.
Spirocysts and ptychocysts are found only in Anthozoa.
basic body plan described above can be manifest as a swimming medusa or attached
polyp. In some taxa only one generation is present whereas in others both are
life cycle featuring alternation of sexual, swimming medusae with benthic
asexual polyps is typical of many cnidarians.
cnidarians are carnivores feeding on live prey which they usually capture using
tentacles armed with cnidocytes. Digestion occurs in the coelenteron which is
typically equipped with ciliated canals for distribution of partly digested
are ammonotelic and diffusion across the body and tentacle surface eliminated
the ammonia from the body. Gas
exchange is across the general body surface. The nervous system is a plexus of
basiepithelial neurons serving sensory and motor systems (Fig 7-6). Most
cnidarians are gonochoric. The
life cycle typically includes a planula larva. Cnidarians
are chiefly marine but the well-known Hydra is
comprises those cnidarians whose life cycle includes a medusa generation that
alternates with a polyp generation (Fig 7-75B). Symmetry
is radial and tetramerous. Nematocysts
are the only type of cnidocyte present. Included taxa are Scyphozoa
(jellyfishes) and Hydrozoa (hydroids, Hydra, Portuguese men of war, etc).
is a diverse taxon of about 3000 species of mostly marine cnidarians. The
life cycle usually includes both polyp and medusa generations (Fig 7-65A) but
may be entirely polyp (Fig 7-65B) or entirely medusa (Fig 7-65C). Polyps
typically are colonial and medusae usually solitary. Some
form colonies of combinations of polyps and medusae. The
few freshwater cnidarians, such as Hydra,
Vallentinia, and Craspedacusta,
polyps are usually small, about 1 mm in length, and colonial. Hydromedusae are
also small, at least in comparison with scyphomedusae, and are usually less than
1 cm in diameter. Hydromedusae
are further distinguished from scyphomedusae by possession of a velum, a
circumferential shelf of tissue that encircles the subumbrellar concavity and
functions as an adjustable diaphragm to create a pulse of water for swimming. Medusae
are tetramerously symmetrical as are scyphomedusae and four radial canals and
four tentacles are usually present.
are found only in the epidermis and germ cells arise in the gastrodermis but
may, or may not, migrate to the epidermis to form gonads and mature into
gametes. Gonads may be on the manubrium or on the radial canals. Gametes, even
if gastrodermal, are released directly to the surrounding water, never, in
contrast with scyphomedusae, into the coelenteron.
polyps and medusa generations in the life cycle (Fig 7-63). Medusae
are thick cup-shaped and large for hydromedusae. The
polyps are colonial, thecate, and tiny but the medusae, which are low and flat,
are sometimes large, up to 50 cm. Statocysts, but not ocelli, are present and
the gonads are on the radial canals.
1. Obelia is
representative of typical hydrozoan morphology and possesses a life cycle in
which polyp and medusa are of approximately equal importance. There
are several species ofObelia and
any can be used for this exercise. If
living specimens are available they should be studied in seawater. Most
laboratories, however, will provide commercially prepared wholemount slides.
a commercially prepared slide on the stage of the compound microscope and focus
Figure 1. Obelia polyp
small portion of a colony. Hydrozoa4La.gif
consist of numerous polyps on branched stalks (Fig 1, 7-64A). A
creeping, rootlike stolon lies
on the surface of a firm substratum and serves as anchor and base for the entire
prepared slides include only a small piece of the colony and the stolon is
rarely present. It
may be absent from living or preserved material also, unless care has been taken
to collect it. Numerous
erect pedicels (=
stem) arise from the stolon. The
stems branch in patterns characteristic of the species and bear polyps.
Obelia polyps (=
zooids) are dimorphic, there being two types in the colony. Gastrozooids are
responsible for feeding, which they accomplish by capturing and ingesting
zooplankton. Gonozooids are
reproductive and produce medusae by asexual budding. Gastrozooids
look like little flowers and have a circle of stinging tentacles surrounding a
mouth, which opens into the coelenteron. Gonozooids are elongate, lack tentacles
or mouth, do not feed, and do not look like flowers.
stems, stolons, and polyps (Fig 1) are enclosed in a transparent, chitinous
exoskeleton called the periderm (=
is a non-living secretion of the epidermis. Inside
is living tissue, collectively known as the coenosarc (pronounced
SIN o sark, coen = common or together, sarc = flesh) consisting of the outer,
nonciliated epidermis and
the inner, ciliatedgastrodermis with
the thin, acellular mesoglea in
three layers can be distinguished by careful focusing with 400X. The
epidermis and gastrodermis are relatively thick and the mesoglea is a narrow
dark line between them.
hollow interior of the animal is the coelenteron,
or gastrovascular cavity. The
cavity extends uninterrupted throughout the colony and is both a chamber for
extracellular digestion and a fluid transport system to deliver partially
digested food through the colony.
gastrozooids provide food for the entire colony, including the nonfeeding
pedicels, stolons, and gonozooids. Find
a gastrozooid and study it carefully. The
periderm of its stem is annulated, or ringed, each ring being an annulus. The
polyp is enclosed in a bell-shaped expansion of the perisarc called the hydrotheca (theca
= a case). Polyps
that are enclosed in this manner are said to be thecate. Obelia,
being thecate belongs to the taxon Leptothecatae. Some
hydrozoan species (Anthoathecatae) have athecate polyps.
coenosarc of the gastrozooid is called a hydranth (=
water flower). It
rests on a transverse shelf of periderm, the diaphragm,
at the base of the hydrotheca. The
coenosarc of the pedicel passes through a small hole in the center of the
diaphragm and is continuous with that of the hydranth. The
distal, or free, end of the hydranth bears a single circle of filiform tentaclesused
to capture prey, sting it, subdue it, and transfer it to the mouth. The
tentacles are solid, contractile, and armed with cnidocytes. Cnidocytes
are clustered together in batteries that
appear as small bumps on the tentacles.
the center of the whorl of tentacles is the erect manubrium,
which bears the mouth at
its center. The
mouth opens into an expanded region of the coelenteron known as the stomach. The
stomach connects with the canal-like coelenteron that extends throughout the
gastrodermis of the stomach contains secretory cells that release proteases. These
enzymes partially hydrolyze food into small particles and molecules for
distribution by ciliary currents to other parts of the colony. Particles
are phagocytized by cells lining the coelenteron and digestion is completed
The gonozooids are
reproductive polyps. They
may be absent from some colonies. They
arise from the pedicel at the bases of the gastrozooids and consist of an
elongate cylinder of perisarc, called the gonotheca,
which, like the hydrotheca, is open distally to the sea. The
gonotheca encloses a cylinder of coenosarc called the blastostyle. The
blastostyle is the living part of the gonozooid. In
mature individuals its surface bears small medusa
buds which are produced
eventually develop into medusae and break free from the blastostyle to swim out
the aperture of the gonotheca into the sea.
medusae, known as hydromedusae, are small, saucer-shaped, free-swimming,
sexually reproducing individuals.
Figure 2. Obelia medusa. Hydrozoa5L.gif
Place a commercially prepared slide of Obelia medusae
on the stage of the compound microscope and focus at 40X. The
medusae of prepared slides are stained a variety of meaningless colors but in
life they are transparent and colorless.
at the slide without the benefit of the microscope and note the size of the
are, in general, much smaller than scyphomedusae and those of Obelia are
even smaller than usual.
saucer-shaped Obelia medusae
are flatter than most hydromedusae but those of commercial preparations are
further flattened by the weight of the coverslip (Fig 2). Most
of the specimens on your slide will probably be lying flat on either the oral or
aboral surface. A
few specimens may be folded and in side view.
have a distinct tetramerous (4-part) radial symmetry with most structures
occurring in fours or multiples of four.
arched saucerlike body is referred to as the bell. It
has a convex upper, exumbrella (aboral
surface ) and
a concave lower, subumbrella (oral
aboral surface is smooth and rounded. In Obelia,
the velum, which is characteristic of most hydromedusae, is rudimentary and is
not visible. Numerous tentacles extend
from the margin of the bell. The tentacles contract when preserved and those of
living specimens would be much longer.
life, the manubrium hangs
like a pendulum into the center of the subumbrellar space below the oral surface
but in wholemounts it is deflected to the side (Fig 2). It
may be necessary to look at several medusae to get a good side view of the
The mouth is
in the center of the free, oral end of the manubrium and is flanked by four
short oral arms. The
manubrium and oral arms are well supplied with cnidocytes. Take
optical sections through the manubrium to convince yourself that it is hollow.
The coelenteron consists
of the stomach at the base of the manubrium, where extracellular digestion
occurs, and a set of ciliated canals used to distribute partly digested food. The
mouth opens into the spacious lumen of the manubrium which in turn empties into
the stomach. The
stomach is in the center of the bell and usually stains darker than the
canals extend from the
stomach to the periphery of the bell where they join the circular ring
canal in the margin of the
radial canals are usually faint and indistinct in commercial preparations but
are visible with careful focusing with good light adjustment. The
mesoglea of the exumbrella is thick and accounts for most of the thickness of
the bell. That
of the subumbrella is thin. Consequently,
the radial canals are easier to see in subumbrellar view.
distinct, darkly staining, epidermal gonad,
either ovary or testis, can be seen below each radial canal near the stomach.
to thirty solid tentacles radiate
from the edge of the bell. The tentacle epidermis has abundant sensory cells and
tentacles of preserved specimens are contracted and much shorter than those of
relaxed living specimens. The
tentacles are often broken in commercial preparations. The
cnidocytes are concentrated in rings, or batteries,
and are used to sting and subdue the prey. A
swollen tentacular bulb in
the base of each tentacle is the site of intracellular digestion and cnidocyte
are eight spherical statocysts spaced
evenly around the margin of the bell. Each
is at the base of a tentacle. The
statocysts are hollow, thin-walled, epidermal vesicles containing a calcareous
statolith and a ciliated sensory epithelium. There
are no ocelli in Obelia or
FA (ed.) 1950. Selected
Invertebrate Types. Wiley,
New York. 597p.
WS. 1958. Practical
Invertebrate Anatomy (2 nd ed). MacMillan,
Ruppert EE, Fox RS,
Barnes RB. 2004.
Invertebrate Zoology, A functional evolutionary approach, 7 th ed.
Brooks Cole Thomson, Belmont CA. 963 pp.
MB, Edwards NC. 1991. Cnidaria:
91-183 in Harrison FW, Westfall JA (eds). Microscopical
Anatomy of Invertebrates, vol. 2. Wiley-Liss,
1 compound microscope
1 wholemount of Obelia polyps
1 wholemount of Obelia medusae