Invertebrate Anatomy OnLine

Aiptasia pallida ©

and  Metridium senile



Copyright 2001 by

Richard Fox

Lander University


            This 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 to 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.  


Cnidaria P, Anthozoa C, Zoantharia sC (=Hexacorallia), Actinaria O, Aiptasiidae F (Fig 7-42, 7-75)

Cnidaria P

            The 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.

            The defining cnidarian feature is, of course, 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 7-10).   Nematocysts (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.

            The 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 found.   A life cycle featuring alternation of sexual, swimming medusae with benthic asexual polyps is typical of many cnidarians.

            All 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 food.   Cnidarians 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 an exception.

Anthozoa C

            This heterogeneous taxon includes stony corals, sea fans, anemones, sea pansies, and others.   Anthozoa is the largest cnidarian taxon, with over 6000 species of exclusively marine polyps.  Anthozoans are always polyps, with no medusa in the life cycle.  The polyps can be solitary or colonial.   The polyps tend to be large, with extremes up to 1 m in diameter, and colonies, such as the corals, may be even larger. The mesoglea contains cells and all three types of cnidae are present, the only taxon for which that is the true.   The mouth opens into a flattened, ectodermal invagination known as the pharynx, which in turn opens into the coelenteron.   Many have endosymbiotic zooxanthellae that figure importantly in their biology.   The coelenteron is compartmentalized by longitudinal septa and gonads are gastrodermal.  

Zoantharia sC

            This, the largest anthozoan taxon with 4000 species, includes sea anemones and stony corals.   The septa that partition the coelenteron are in multiples of six and usually occur in pairs. The gonads are in the septa.  Cnidae consist of nematocysts, spirocysts, and ptychocysts with spirocysts and ptychocysts being found nowhere else.    

Actiniaria O

            Actiniaria, the sea anemones, are solitary and have the morphology typical of anthozoan polyps (Fig 7-20). Anemones are relatively large polyps, compared with hydrozoans and scyphozoans, with most being less than 10 cm long and 5 cm in diameter but some are much larger.   Anemones can dramatically alter their size and shape by inflating (with seawater) or deflating (Fig 7-19). About 1350 species are known.   Most anemones attach to a firm substratum by the pedal disc (Fig 7-22) but many inhabit soft substrata and do not attach (Fig 7-21).   The periphery of the coelenteron is partitioned by abundant longitudinal, sheetlike septa which greatly increase the surface area for absorption of food and its subsequent intracellular digestion.  The central region of the cavity is unpartitioned and is the site of extracellular digestion.   The free edges of the septa border the central region and secrete hydrolytic enzymes into it.   Polymorphism and alternation of generations are absent.   Anemones are gonochoric or consecutive hermaphridites.  

Laboratory Specimens

            Almost any sufficiently large species of anemone can be used for the study of basic anthozoan anatomy.   The exercise is written for Aiptasia pallida with parenthetical references toMetridium senile.   Due to the homogeneity of Actiniaria it will apply, with few discrepancies, to other species.   Aiptasia is available alive from supply companies but at present is not offered preserved.   Metridium is available alive and preserved.                                                     

            Aiptasia pallida is a common anemone on floating docks, oyster reefs, and other hard substrata in shallow water in the southeastern United States.   It occurs in two colors, one white and the other a dark rich yellowish-brown.   The color is due to the presence of endo­symbiotic dinoflagellates (zooxanthellae) in the gastrodermal cells.   It is best to use dark individuals if they are available.  

            Metridium is widespread and occurs on the coasts of northeastern North America and northern Europe.   It inhabits shallow, subtidal water and lives attached to rocks.   It is larger thanAiptasia and does not harbor zooxanthellae.  

            If there is a choice, Aiptasia, or another relatively thin-walled species, is a better subject for dissection than is Metridium.   The latter is difficult to relax in an expanded condition and dissection of the contracted animal is frustrating and confusing.   Preserved specimens from supply houses are often in the contracted condition.  

            Living specimens are far! superior to preserved for the study of anemones and should be used if at all possible.   Living specimens should be relaxed by immersion in isotonic magnesium chloride shortly before the laboratory period and they should be kept in this solution during the dissection.   Preserved specimens should be dissected in tapwater.   (If preserved specimens of Metridium are to be used, it would be better to follow the companion exercise for Metridium, as it is written specifically for preserved specimens of that species.)

            The dissecting microscope should be used for most of the dissection.   If possible, a few anemones should be maintained in an aquarium in the laboratory for observations of life position, behavior, and natural shape.   If possible, there should also be living unrelaxed specimens in a fingerbowl for each student.   These specimens will not be damaged and can be reused.

            Take a dish or small dissecting pan containing a relaxed living or a preserved specimen to your bench.   Examine the specimen with the dissect­ing microscope.  If they are available, take a dish with an unrelaxed living specimen to your desk also.   Handle he unrelaxed specimen gently and avoid sudden movements.   You will see more if it does not contract.   Avoid spilling seawater or magnesium chloride on the microscope.  

External Anatomy

            The body of a typical anemone is a cylindrical column with a crown of tentacles at one end and an adhesive pedal disc at the other (Fig 1, 7-16A).   The column is hollow and contains the coelenteron.  

            The two ends of the column are discs.  The attached, basal end of the column is the aboral end, which terminates in the pedal disc.   It is usually attached to some firm substratum such as rock or shell.  

            The opposite end is free in the water and is the oral end terminating in the oral disc.  The slit-shaped mouth lies in the center of the oral disc. In Metridium (and Actinia, Anemonia, andTealia) there is a circumferential fold, or collar, around the column that divides the column into two regions.   The collar overhangs a deep fold, the fossa.   The division is not so clearly marked in Aiptasia where there is no collar or fossa.   

Figure 1.   The anemone, Aiptasia pallida, with a quarter of the column cut away to reveal the interior.   The tentacles and septa closest to the observer are omitted for clarity.  Anthozoa8L.gif

Figure 1

            Whorls of unbranched tentacles encircle the margin of the oral disc.   The tentacles are hollow evaginations of the body wall and contain extensions of the coelenteron.   Their epidermis contains cnidocytes used for stinging either prey or predators.   The tentacles of Aiptasia are large and relatively few in number whereas those of Metridium are smaller and more numerous.  

            The mouth is an elongate slit in the center of the oral disk.   It is expanded a little at each end.   These expansions are the ends of vertical, ciliated grooves called siphonoglyphs (Fig 7-16B).   The cilia of the siphonoglyphs generate a constant flow of water into the coelenteron even when the mouth is closed.   This maintains a positive hydrostatic pressure in the cavity and helps keep the column turgid and erect.   The mouth opens into a short, flattened tube, the pharynx, that itself opens into the coelenteron (Fig 7-16).   The siphonoglyphs are at opposite edges of the flat pharynx.   You cannot see the pharynx yet.

            The coelenteron has many functions.   It is the digestive chamber, of course, but is also the fluid transport system.   The ciliated gastrodermal cells lining the cavity circulate partly digested food throughout the cavity thus supplying all the cells of the body.   Extensions of the cavity occupy the tentacles.   The cavity also functions as a hydrostatic skeleton against which the circular and longitudinal muscles of the body operate.   It is also a chamber for maturation of gametes and reception of nitrogenous wastes.  

            >If possible, use transmitted light to focus on the surface of the oral disk of an unanesthetized and expanded living specimen.   Look for moving particles in the coelenteron.   If you wish, use a syringe to inject some carmine/seawater through the mouth into the coelenteron.   Watch for circulation of the carmine particles throughout the cavity, including the interiors of the tentacles.<

            Look carefully at the lower part of the column for the small blister-like cinclides through which the animal expels acontia.   You may be able to see the long, threadlike acontia through the body wall but will probably have to wait until you open the animal to see them.   The epithelium of acontia bears large numbers of cnidocytes.   (Many anemones do not have acontia or cinclides but Aiptasia and Metridium do.)  

            >If a living, unrelaxed specimen is available, aggravate it with a blunt probe while watching the cinclides under low power.   Continue prodding the animal until it expels acontia through the cinclides (and mouth as well).   You may be able to see the cinclide gape just before the acontium streams through.   What mechanism expels the acontia?   How do you suppose acontia are able to find the cinclides so that they can pass through them?   Set this specimen aside and let it re-expand. <

            The flat pharynx and the linear mouth with a siphonoglyph at each end destroy the otherwise perfect radial symmetry of anemones and confer upon them a biradial symmetry.   Such a body plan has two planes of symmetry rather than the several planes of a radially symmetrical animal or the single plane of a bilaterally symmetrical one.   Both planes of symmetry are longitudinal and pass through the discs.   One passes through both siphonoglyphs whereas the other is perpendicular to the first and passes midway between the two siphonoglyphs.   Some anemones have only one siphonoglyph and are thus bilaterally symmetrical with only one plane of symmetry.   Aiptasia, and usually Metridium, have two siphonoglyphs, although someMetridium have one or even three of them.  

Figure 2. Make sketches of oral disks to show how the shape of a straight mouth with two siphonoglyphs imposes biradial symmetry on the oral disk and how a single siphonoglyph results in bilateral symmetry. Anthozoa76L.gif

  Figure 2

            >Apply carmine/seawater to the outside of the oral disk of an unrelaxed, expanded specimen and watch the motion of the particles in the vicinity of the siphonoglyphs.   Record your observations. <

            If your Aiptasia came from a clean, well lighted place, it will be dark brown.   The color is due to autotrophic symbionts in the cells lining the coelenteron of the tentacles and column.  These are zooxanthellae, or dinoflagellates that have lost their flagella.   This species is Symbiodinium microadriaticum and it is symbiotic with a variety of cnidarians.   Metridium does not have zooxanthellae.

            >Snip the tip from one of the tentacles of a living Aiptasia and transfer it to a drop of seawater or magnesium chloride on a slide.   Using the dissecting microscope, affirm the presence of a thick dark brown (in life) layer of cells on the inner side of the tentacle wall.   Tease apart the tentacle and spread its cells about on the slide.   Add a cover slip and examine it with the compound microscope at 400X.   The brown layer is composed of abundant spherical  zooxanthellae.   Indicate their color on your drawing.   Consider the benefits to each partner in this mutualistic association.   What does each gain by participation in the association? <

            Return once again to your preserved or relaxed anemone.   If relaxed, push it roughly with the probe and see if it can still contract its longitudinal muscles.   When the specimen no longer responds to your touch, you may begin the dissection.   Place the specimen, whether relaxed or preserved, it in a small wax-bottom dissecting pan if it is not already in one.  

Internal Anatomy

"     Insert the point of a pair of fine scissors through one of the siphonoglyphs of the mouth into the pharynx and cut through the body wall in the plane passing through the siphonoglyphs.  Cut all the way from the oral to the pedal disc.   Observe that the upper end of the animal consists of a smaller tube (the pharynx) within a larger tube (the body wall).   The space between is thecoelenteron.   The pharynx extends only part of the way down the column and is connected with the body wall by sheets of tissue.   Make a second, similar, cut through the second siphonoglyph opposite the first and lay the animal open.   Separate the two sides of the animal and pin them to the wax of the dissecting pan. Be sure the specimen is completely covered with fluid. "

            A set of at least 12 complete septa (= mesenteries) extend from the body wall across the coelenteron to the pharynx (Fig 1, 7-16).   These are thin sheets of tissue composed of a double layer of gastrodermis with a thin layer of mesoglea sandwiched between.   Epidermis is not present in the septa. The number of septa may be greater but they occur in multiples of 12.  There are never less than six pairs of complete septa.   By definition, complete septa are attached to the body wall along one edge and to the pharynx along part of the other edge.   Incomplete septa are attached only to the body wall.

            Below the level of the pharynx, the complete septa are unattached along their inner edges.   The free, unattached, inner edges of the septa are thickened and are called septal filaments(Fig 1, 7-16).   Each filament is composed of three side by side lobes running the length of the filament from the end of the pharynx to the pedal disk (Fig 7-17).   They are best developed near the pharynx.   Despite their name they are not filamentous but are instead simply the thickened edge of a mesentery.   They are longer than the distance between the end of the pharynx and the pedal disk and consequently are wavy and folded on themselves, especially near the pharynx.   Of the three filamentary lobes, the two outer lobes are ciliated and colorless.   The middle lobe is not ciliated but is glandular and secretory.   It bears cnidocytes and in Aiptasia is brownish.

            Closely examine the aboral end of a filament with the highest power of the dissecting microscope.   You will be able to see clearly the three lobes of which it is composed.   Consult your textbook to review the functions of the three lobes.

            Acontia arise from the aboral end of the middle lobe of the septal filaments.   There is one acontium per septum.   Much of the space in the coelenteron is filled with the filaments and the long thread-like acontia they bear.   Follow the free edge of a septum down into the column, noting that it becomes less convoluted near the pedal disk.   The acontia arise from the lower, unconvoluted portion of the septum.   Acontia are packed with cnidocytes.  

            In addition to the complete septa there are numerous incomplete septa that arise from the inner wall of the column (as do the complete septa) but do not extend all the way across the coelenteron and do not join the pharynx.   Look at the spaces between the complete septa and find some of the narrower incomplete septa arising from the body wall.

            >Place carmine particles on the exposed surface of the pharynx and septa and watch the movement.   Describe the path taken by the particles. <

"     Snip off a piece of an acontium and make a wetmount in seawater or magnesium chloride and set it aside until it is convenient to study it. "

            >When convenient, place the wet mount on the stage of the compound microscope and examine the acontium with 400X with the light carefully adjusted.   Notice the cilia on the margins of the acontium.  

            With a piece of absorbent paper, remove some of the liquid from beneath the coverslip thereby flattening the acontium so that light can pass through it.   You should now be able to see clearly an outer layer of long, fusiform nematocysts.   Nematocysts are not entire cells but are the largest and probably the most elaborate organelles known.   The cells themselves (cnidocytes) are not visible.  

            If you withdraw too much water the nematocysts will be squeezed out of the acontium.   No harm is done if this happens, in fact it is desirable to have several nematocysts isolated from the acontium itself.   You should be able to find two different types of nematocysts, one large and one small.  

            The nematocyst consists of an outer, fusiform capsule in which is coiled a slender tubule, or thread.   Look closely for the eversible tubule coiled within the undischarged nematocyst.

            If your acontium came from a living specimen, draw a drop of 1% acetic acid under the coverslip.   Watch carefully through the microscope as you do this.   The acid will stimulate the discharge of the nematocysts.   Observe the response of the nematocysts when the acid comes in contact with them. Find a field where both discharged and undischarged large nematocysts are present and examine both.   Compare the discharged and undischarged nematocysts.   In the undischarged capsule you can see the tubule coiled in the storage condition.   In the discharged capsule you can see the fully extended barbed shaft with the long thread at the end. <

            Look again at your dissected specimen.   Spread and flatten one of the complete septa as much as possible and look for its retractor muscle.   This is a longitudinal band of muscle running along one surface of the mesentery from oral to pedal disc in about the middle of the mesentery (Fig 1, 7-18A,B).   Contraction of these muscles shortens the column and withdraws the tentacles and oral disk into the column.   There is also a circumferential sphincter muscle of circular fibers around the oral end of the column to close the oral end over the retracted tentacles.

            Anemones also have circular muscles in the body wall of the column.   Their contraction pressurizes the coelenteron.   In addition, the complete septa contain radial muscles.   These run from pharynx to the body wall and open the pharynx and mouth when they contract.   During contraction, the anemone opens the mouth and contracts the longitudinal muscles, expelling water from the mouth (and cinclides).

            >If you have an expanded, unrelaxed specimen in a dish, stimulate it with a gentle push from a probe or applicator stick.   Watch its response.   Is there evidence of contraction of longitudinal muscles?   Can you see the mouth open?   What happens to the volume of the animal? <

            Aiptasia and Metridium reproduce asexually by pedal laceration.   Small masses of cells are pinched off the margins of the pedal disk (Fig 1, 7-22).   These grow slowly and differentiate into small anemones.   An aquarium containing healthy Aiptasia will soon have numerous small pedal lacerates on its walls.   Ask the instructor if pedal lacerates are available in the laboratory and examine them if so.

            Most anemones are gonochoric although some are hermaphroditic.   Since there are no medusae, the polyps are sexual and bear the gonads. These are longitudinal gonadal bands of gamete-producing tissue on the edges of the septa just peripheral to the septal filaments and central to the longitudinal muscles (Fig 7-18A).   The gonadal bands are usually located between the longitudinal muscle and the septal filament on incomplete septa.  Fertilization may be internal, in the coelenteron, or external, in the sea.   Upon settling, the planula develops into a new polyp.

            If your specimen is living, remove it from magnesium chloride and discard it or, better, put it in a seawater aquarium.   Do not pour the magnesium chloride in the aquarium.   Aiptasia has remarkable regenerative powers and the dissected specimen will recover and put itself back together in a few days.   Visit the aquarium periodically over the next several days and watch their recovery.   They can be maintained on small pieces of shrimp or fish.

*Hyphenated call-outs, such as this one, refer to figures in Ruppert, Fox, and Barnes (2004).   Those without hyphenation refer to figures embedded in this exercise.


            Barham EJ, Pantin CF .   1951.   The organization of the muscular system of Metridium.   Quart. J. Micros. Sci. 92:27-54.

            Fautin DG, Mariscal RN. 1991.   Cnidaria: Anthozoa, in Harrison, F. W. &   J. A. Westfall (eds.). Microscopic Anatomy of Invertebrates vol. 2 Placozoa, Porifera, Cnidaria, and Ctenophora.   Wiley-Liss, New York.  436p.

            Hyman, L. H.   1940.   The Invertebrates: Protozoa through Ctenophora, vol. I.   McGraw Hill, New York.   726p.  

Ruppert EE, Fox RS, Barnes RB.   2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.  

            Stephenson TA.   1928.   The British Sea Anemones.   Ray Society.  

            Waterman TH.   1950.   Metridium senile in, F. A. Brown (ed), Selected Invertebrate Types.   Wiley, New York.   pp 119-127.  


1 large living Aiptasia or Metridium

1 dissecting microscope

1 compound microscope

1 dissecting pan approximately 10 X 6 X 3 cm, with wax bottom (e.g. sardine can)

slides and coverslips

isotonic magnesium chloride


1% acetic acid


            Living Aiptasia are available from Gulf Specimen Marine Laboratories, Metridium from Woods Hole Marine Biological Laboratory.