Invertebrate Anatomy OnLine

Cucumaria frondosa ©

Sea Cucumber


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, 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.  


Echinodermata P, Eleutherozoa, Cryptosyringida, Echinozoa, Holothuroidea C, Dendrochirotacea sC, Dendrochirotida O, Cucumariidae F (Fig 9-26, 27-12, 28-62)

Echinodermata P

            Echinoderms 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 their 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 and the fossil record includes 13,000 extinct species.

            An important echinoderm apomorphy is the water vascular system that in most taxa 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 that assumes different forms in different taxa.   In 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.

            Excretion 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 echinoderms.   A 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 compartments.   The 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.  


            Eleutherozoans 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 the sister taxon, Crinoidea.


            Cryptosyringida includes Ophiuroidea, Echinoidea, and Holothuroidea, all with closed ambulacra in which the radial nerve is internalized and protected by ossicles (Fig 28-20).


            Echinozoa, sister taxon to Ophiuroidea, consists of 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 around 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

Holothuroidea C

            The holothuroids, or sea cucumbers, are echinoderms in which the body is elongated along the aboral/oral axis.   The body wall is thick and well developed.   It consists of a nonciliated epidermis, connective tissue dermis, circular and longitudinal muscles, and a ciliated peritoneum.   It is unusual in that its longitudinal muscles are in five longitudinal bands and the ossicles are microscopic plates scattered in the dermis.   Abundant collagen fibers in the mutable dermis can, under nervous control, be cross-linked with each other to form a rigid skeleton that can scarcely be cut with a scalpel.

            The gut is a simple tube extending from mouth to anus.   Gas exchange is via diverticula of the posterior gut.  Large branched tube feet encircle the mouth and are used for feeding.   The hemal system is better developed than is typical in echinoderms and the coelom is large and spacious.   The madreporite opens into the perivisceral coelom, rather than to the outside.   Sea cucumbers are gonochoric and have a single gonad opening to the exterior via a gonopore at the oral end.  

Dendrochirotida O

            The body of dendrochirotes is more or less cucumber-shaped and the buccal tube feet are branched, or dendritic, and have no ampullae. Most taxa have a retractable introvert.

Laboratory Specimens

            Cucumaria frondosa (Fig 28-45) is a northern Atlantic species occurring as far south as southern New England and on the coasts of northern Europe.   This exercise is intended for use with preserved Cucumaria.  If living specimens are available, the exercise for Sclerodactyla should be used instead .

External Anatomy


            The body is elongate with the mouth at the oral end and the anus at the opposite, aboral, end. A weak bilateral symmetry is superimposed on the underlying echinoderm radial symmetry. The aboral-oral axis is the long axis of the body and is the axis of symmetry, both bilateral and radial. Cucumbers are radially symmetrical but also possess a superficial bilateral symmetry in which the oral end is also the anterior end. The aboral end is also the posterior end.


            The anterior (oral) end bears mouth and a circle of ten branched tentacles, the buccal podia (Fig 28-45).   It is an introvert that can be fully retracted into the body by a set of powerful retractor muscles.   Threatened animals or those preserved without adequate relaxation may have retracted introverts, in which case the tentacles and mouth will not be apparent externally.   If the tentacles are not visible in your specimen, try to evert them before you begin the dissection.   Sometimes this can be accomplished by gently squeezing the body.   If not, you will have to wait until you open the body cavity to see them.


            In Echinozoans (urchins and cucumbers) the ambulacra arise around the mouth at the oral end, which is also the oral ambulacral center, and extend aborally to the aboral ambulacral center, which in cucumbers is the posterior end. The five ambulacra are indicated externally by five longitudinal rows of small, inconspicuous tube feet (Fig 28-45).   The areas between the ambulacra are the interambulacra (Fig 1).                                                                            

            The longitudinal body wall muscles of cucumbers do not occur in a continuous sheet rather are concentrated in five strong longitudinal muscle bands.   These bands are ambulacral in position and can sometimes be seen externally.  

            Most cucumbers creep over the surface of the substratum or burrow into it.   A (very) few swim (Fig 28-49B,C).   When creeping they keep the ventral surface in contact with the bottom and this surface is specialized for the purpose.   The flat ventral surface has better developed tube feet, which are known as locomotory podia (Fig 28-45).   They usually have suckers.   Three of the five ambulacra are associated with the ventral surface and are together known as the trivium.   The two dorsal ambulacra are the bivium.  


            Review the orientation of the cucumber and be sure you can find anterior (oral), posterior (aboral), dorsal (bivium), ventral (trivium), right, and left.  

            The dorsal midline coincides with an interambulacrum between the two ambulacra of the bivium, whereas the ventral midline is on the middle ambulacrum of the trivium (Fig 1).   If a cross section of a cucumber is compared to a clock face, the ambulacra would be located at roughly 2, 4, 6, 8, and 10 o'clock.   The five ambulacra are referred to as right dorsal (2:00), left dorsal (10:00), right ventral (4:00), left ventral (8:00), and ventral (6:00).   Similarly, the interambulacra are dorsal, right dorsal, left dorsal, right ventral, and left ventral.  

Figure 1.   A diagrammatic cross section of a typical cucumber showing the ambulacra, mesenteries, and gut loops.   A = ambulacrum, I = interambulacrum,   R = right, L = left, D = dorsal, V = ventral. Cuke11La.gif

Figure 1


            Find the mouth at the center of the anterior end surrounded by a narrow peristomial membrane.   The ten buccal podia are large, branched tube feet, one pair of which is associated with each ambulacrum.   A single small genital papilla lies between the trunks of the two dorsal podia and bears the gonopore (Fig 28-45).  


            The opposite, posterior (aboral) end of the cucumber has the anus at its center (Fig 28-45).

Internal Anatomy

"     Perform the dissection with the specimen immersed in tapwater (or isotonic magnesium chloride if living) using magnification as required.   Open the body cavity, or perivisceral coelom, by making a longitudinal incision from the anus anteriorly in the right dorsal interambulacrum (Fig 1).   Use strong scissors and cut completely through the thick body wall without damaging the tissues in the coelom.   Avoid cutting along or across the dorsal midline to avoid damaging the dorsal mesentery located there.   Extend the incision to the bases of the buccal podia.  Pin the body wall aside with # 4 stainless steel insect pins so you can see the body cavity and its viscera.   The body wall of preserved specimens will be stiff and contracted so you may have to make additional cuts before you can pin the wall against the wax.  These cuts should be perpendicular to the original incision and should be made to the right of it.  


            The body cavity, or perivisceral coelom, is large and spacious (Fig 2, 28-50).   Locate its major organs to use as landmarks.   One of the most conspicuous features of the coelom is the white calcareous ring at the anterior end at the base of the tentacles.   The orange gonad is a large tuft of long slender filaments that may fill much of the coelomic space, obscuring other structures.   The gut is a translucent or opaque tube that loops its way from the anterior mouth to the posterior anus.   Most of it is supported by mesenteries.   The two respiratory trees are large, delicate, translucent, branched tubes.   The five longitudinal muscle bundles can be seen on the inside of the body wall and mark the positions of the ambulacra.   From the muscles arise five introvert retractor muscles that cross the anterior body cavity to insert on the calcareous ring.

Body Wall

            Look at the cut edge of the body wall and note the thick layer of collagenous connective tissue.   Most of the thickness of the wall is the connective tissue dermis.   There is a thin epidermis outside the dermis. Calcareous ossicles, which are present in the outer layer of the dermis of most cucumbers (Fig 28-47) are scarce in Cucumaria.  

            >1a . If available in the laboratory examine permanent slides of cucumber dermal ossicles using the compound microscope.    If such slides are not present, ossicles can sometimes be found by digesting a bit of a buccal podium in a drop or two of bleach on a slide.   Make a wet mount and examine it with the compound microscope. This is usually unsuccessful withCucumaria. <

            The five longitudinal muscles are easily seen on the inner surface of the body wall (Fig 1).   They are ambulacral in position and run from the posterior end of the body to insert on the middle of the calcareous ring.   The introvert retractor muscles are extensions of the longitudinal muscles and also insert on the calcareous ring.   Contraction of the retractor muscles pulls the calcareous ring into the coelom. The anterior end of the body, which is the introvert, is attached to the ring and is pulled in also.   Contraction of circular body wall muscles pressurizes the coelomic fluid and extends the introvert.

Calcareous Ring

            A calcareous ring of 10 large ossicles, five ambulacral and five interambulacral, embedded in white connective tissue encircles the anterior end of the gut (Fig 2).   It supports the ring canals of the water vascular and hemal systems, provides a mechanical base for the buccal podia and introvert, and is the site of insertion of the longitudinal body wall muscles and the introvert retractor muscles.


            Of the many subdivisions of the echinoderm coelom the water vascular system and the body coelom are the two most obvious.   The body coelom is subdivided into a large posteriorperivisceral coelom, which contains most of the viscera and is the chief body cavity, and the peripharyngeal coelom surrounding the calcareous ring.   The calcareous ring and peripharyngeal coelom are enclosed in a mesenterial sac which, along with its contents, is the peripharyngeal bulb.   The peripharyngeal coelom is not visible to you now but the perivisceral coelom and peripharyngeal bulb are.  

Figure 2.   Dorsal dissection of Sclerodactyla.   The incision was made along the right dorsal interambulacrum.   The left respiratory tree and hemal system omitted.   All introvert retractor muscles except the right ventral muscle have been omitted to simplify the drawing.   Abbreviations as in Fig 1.   Based on Coe, 1912. cuke12La.gif

Figure 2

            The perivisceral coelom is incompletely partitioned by three longitudinal mesenteries.   They are thin, transparent double sheets of peritoneum that extend from the body wall into the perivisceral coelom.   They surround and support some of the visceral organs, especially the gut (Figs 1, 2).   The dorsal mesentery runs along the dorsal midline on the dorsal interambulacrum (Fig 1).   The left mesentery is in the left dorsal interambulacrum.   The right mesentery is in the right ventral interambulacrum.   Each of the three mesenteries is attached to a different region of the gut. Coelomocytes of many types are present in the coelom.  

Reproductive System

            The gonad is associated with the dorsal mesentery (Fig 1, 2, 28-50).   Holothuroids are gonochoric but show no anatomical sexual dimorphism and the tissues of the gonad must be examined microscopically to determine sex.   In contrast to all other echinoderms, the reproductive system of cucumbers is bilaterally, not radially, symmetrical. Only one gonad is present but it consists of two tufts of long, spaghetti-like, tubes whose free ends may extend throughout much of the coelom.   It is on the dorsal midline in the anterior perivisceral coelom.   The gonoduct(vas deferens or oviduct depending on sex) arises from the gonad and runs anteriorly along the dorsal midline.   It is a distinct white tube connecting with the gonopore on the genital papilla.  

            With the animal completely immersed in liquid, gently rake the filaments of the right tuft of the gonad together into a tidy bundle resembling a mop head.   This will make it easier to observe its organization.  Look at the anterior edge of the bundle where you will see much smaller, less heavily pigmented immature filaments.   The number of filaments increases as the animal grows with new ones appearing anteriorly.  

"     The filaments of the gonad may be removed if they obscure the remaining viscera. Hold the clump of filaments together and cut through the base of the tuft with scissors.   Discard the filaments.   Do not leave them in your pan or they will become entangled with everything else.   Do not damage the dorsal mesentery, respiratory tree, or the associated gut.   Remove the left tuft using the same technique.   If the mesentery makes it difficult to gather the second tuft into a bundle, you may have to remove a few filaments at a time.   Check to be sure the branches of the respiratory tree are not entangled in the filaments before you cut.

Digestive System

            Move the two respiratory trees aside so you can see the gut tube, noting the close association between the left respiratory tree and the gut as you do.  

            The mouth opens into the pharynx which passes through the center of the calcareous ring where it cannot be seen.   The esophagus exits the pharynx and calcareous ring to enter the perivisceral coelom (Fig 2, 28-50).   It soon joins the short stomach but in Cucumaria the regions of the anterior gut are poorly defined and difficult to distinguish. 

            The very long intestine exits the stomach and eventually empties into the cloaca at the posterior end of the coelom.   The intestine makes two turns and is composed of three regions. They are, in order, the descending anterior intestine, ascending anterior intestine, and posterior intestine (= rectum).  

            The descending anterior intestine runs posteriorly from the stomach and is suspended from the body wall by the dorsal mesentery (Figs 1, 2).   In the posterior part of the body it reverses direction and becomes the ascending anterior intestine.   This region of the intestine runs anteriorly attached to the body wall by the left mesentery.   At the anterior end of the body cavity the intestine reverses direction again and becomes the posterior intestine.   The posterior intestine runs posteriorly attached to the body wall by the right mesentery.  

            The posterior intestine joins the short, wide cloaca in the extreme posterior end of the body.   This terminal region of the gut is attached to the surrounding body wall by an abundance of threadlike radial cloacal dilator muscles.   The opening of the intestine into the cloaca is small and kept tightly closed by the intestinal sphincter.   The cloaca opens to the exterior via the anus, which is closed by the anal sphincter.   The respiratory trees extend from the cloaca into the coelom.

            >1b . Open the cloaca with a longitudinal incision and find the opening of the intestine and its sphincter in the anterior end.   Also find the openings to the two respiratory trees on the lateral walls of the cloaca. <

Respiratory System

            The major respiratory organs are the two respiratory trees.   These are hollow, highly branched evaginations of the wall of the cloaca.   They extend into the perivisceral coelom and are surrounded by coelomic fluid.   The trees are anchored to the body wall by fine connective tissue threads and the left respiratory tree is intimately associated with the intestine.

            >1c . Snip off a piece of the respiratory tree and place it in a small culture dish of tapwater.    Observe it with transmitted light on the stage of a dissecting microscope at high power (30-40X).   Look for the thick layers of circular and longitudinal muscles in its walls.   Find the cut end and verify that the branches are hollow.   <

            To inhale the cucumber closes the intestinal sphincter, relaxes the anal sphincter, and contracts the cloacal dilator muscles.   This expands the cloacal lumen and inspires water into the cloaca.   Next both sphincters are closed and the circular muscles in the cloacal wall are contracted forcing water into the respiratory trees. Gases are exchanged between coelomic fluid and seawater across the thin walls of the trees.   Water is exhaled by contraction of muscles in the walls of the trees (with intestinal sphincter closed, anal sphincter open).   

Water Vascular System

            The ring canal (water ring) is located in the peripharyngeal bulb at the posterior end of the calcareous ring (Fig 2).   It is a large, easily seen, membranous doughnut with thin, translucent walls.   The esophagus passes through the hole in the doughnut.

            The small stone canal (= madreporic canal) arises from the anterior dorsal wall of the ring canal and extends a short distance into the perivisceral coelom to end at a small knotlikemadreporite attached to the dorsal mesentery beside the gonoduct (Fig 2).   Note that the madreporite opens into the perivisceral coelom and not to the exterior.   Its surface is covered by a ciliated epithelium through which pores open into the stone canal.

            Also opening from the ring canal is a single Polian vesicle.   This thin-walled, hollow sac is a diverticulum of the ring canal filled with the fluid of the water vascular system.   Its function is not well understood but it may serve as a fluid reservoir which is used to pressurize the water vascular system.   The Polian vesicle of Cucumaria arises on the left of the dorsal midline and is very long, extending to the posterior end of the coelom.   It is highly distensible and may be slender when empty or grossly swollen when distended.   No Tiedemann's bodies are present.

            The five radial canals of the water vascular system arise from the anterior edge of the ring canal and initially extend anteriorly.   They are large where they leave the ring canal and then widen into even larger translucent radial reservoirs between the ring canal at the posterior end of the calcareous ring.   They then continue anteriorly, through the calcareous ring (out of sight) to emerge anterior to the calcareous ring.   Each canal then reverses direction and extends posteriorly on the body wall under a longitudinal muscle bundle.   The canal is hidden by the muscle. The radial water canals are supported by the ambulacral ossicles of the calcareous ring and the canals to the buccal podia lie between adjacent ambulacral and interambulacral ossicles.

            >1d . Make a transverse section through the right dorsal ambulacrum and find the radial water canal between the longitudinal muscle and the body wall. <

            In the body wall the radial canal gives rise to numerous lateral canals but these are hidden in the body wall musculature.   Each lateral canal leads to the ampulla of a tube foot.   The ampullae are small, elongate, transparent sacs on the inner surface of the body wall.   The ampullae are in neat rows on either side of the longitudinal muscle bundle of the ambulacrum.

            The podia are cylindrical extensions of the body wall visible on the outside surface.   Each contains a blind extension of the water vascular system connected to an ampulla and lateral canal.  

            Two buccal podia are associated with each of the five radial canals.   Each has an ampulla with a lateral canal leading to it but the ampullae are quite large.   They are long, translucent sacs anterior to the calcareous ring, near the insertions of the retractor muscles.   The two ampullae of the two buccal podia of each radial canal flank the insertion of the retractor muscle of that particular ambulacrum and also flank the site of emergence of the radial canal from the peripharyngeal mass.   The lateral canals to these two big ampullae exit the radial canal just after it (the radial canal) exits the radial reservoir.

Hemal System

            The holothuroid hemal system resembles that of other echinoderms but is better developed.   No axial complex has been found in holothuroids. It includes a dark, brownish hemal ring at the base of the water ring on its posterior face.   It is a circular tube like the water ring but its diameter is much less.

            Five radial hemal canals arise from the hemal ring and accompany the radial water canals along the ambulacra.   The origins of the radial hemal canals can be seen by opening the ring and looking at its walls.   They can also be seen in the cross section made earlier of the ambulacrum.   Two intestinal vessels arise from the hemal ring and run posteriorly on opposite sides of the gut tube.  

            A complex set of branching and anastomosing canals from the intestinal vessel of the descending intestine crosses the intervening coelomic space to connect with the intestinal vessel of the ascending intestine (Fig 28-51).   This intestinal system includes a portal system of capillaries in the intestine and is called the rete mirable (= miraculous net).   It is better developed in aspidochirotacean cucumbers, such as Holothuria, than in dendrochirotes such as Cucumaria but its small transparent vessels are easily seen, even in Cucumaria.   The left respiratory tree is closely associated with the rete.

Nervous System

            The nervous system arises from a nerve ring in the peristomial membrane.   Five radial nerves exit the ring on the ambulacral axes.   They accompany the radial water and radial hemal canals and are peripheral to both, being intraepidermal.   They can be seen in the cross section made earlier to reveal the radial water canal.   There is a nerve plexus in the dermis.

            Chemosensory and mechanosensory receptors are present in the epidermis and holothuroids exhibit a general sensitivity to light but have no special photoreceptive organs.


Abbott DP.   1987.   Observing Marine Invertebrates.   Stanford Univ. Press, Stanford.   380.

            Bullough WS .   1958.   Practical Invertebrate Anatomy (2nd ed).   MacMillan, London.   483p.  

Coe WR.   1912.   The echinoderms of Connecticut.   Bull. Conn. State Geo. Nat. Hist. Surv.   19:1-152.

            Edwards CL.   1910.   Cucumaria frondosa.   Zool. Jahrb. Abt. System 29:333-358, pl. 13.

            Hyman LH .   1955.   The Invertebrates: Echinodermata, vol. IV.   McGraw-Hill, New York.   763pp.

            Lawrence J.   1987.   A Functional Biology of Echinoderms.   Johns Hopkins, Baltimore.   340p.

            Reid WM.   1950.   Thyone briaerius, pp 541-546 in Brown FA. (ed.)   Selected Invertebrate Types.   Wiley, New York.  597p.

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

            Smiley S. 1994. Holothuroidea, pp. 401-471 in Harrison FW, Chia FS (eds.).   Microscopic Anatomy of Invertebrates vol. 14 Echinodermata.   Wiley-Liss, New York.   510p.


Dissecting microscope

Compound microscope

Slides, coverslips

Dissecting pan

Preserved Cucumaria

# 4 stainless steel insect pins