Invertebrate Anatomy OnLine

Ciona intestinalis ©

Sea squirt


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.  


ChordataP, Tunicata sP, Ascidiacea C, Enterogona O, Phlebobranchia sO, Cionidae F (Fig 27-12, 29-32)

Chordata P

            Chordata is characterized by a suite of apomorphies including a dorsal hollow nerve cord, notochord, pharyngeal gill slits, and a post anal tail (Fig 29-1).   The ancestor was a fishlike deuterostome that swam using alternating contractions of right and left longitudinal axial muscles to create undulations of the body.   The flexible, incompressible notochord prevented these contractions from compressing the body while allowing lateral deflection.   The chordate central nervous system is a hollow, median, longitudinal nerve cord formed in the embryo by an invagination of surface ectoderm whose original function was probably sensory reception. Paired pharyngeal gill slits connect the lumen of the pharynx with the exterior and originally functioned in suspension feeding with respiration being added later.   A muscular tail posterior to the anus is, although commonplace in chordates, an unusual feature not found in other taxa.   It is an extension of the axial musculature and is the chief locomotory organ. An additional apomorphy is the endostyle, a region of pharyngeal endoderm, that secretes iodated compounds, either mucus or hormones.   

Tunicata (= Urochordata) sP

            Tunicates are highly derived and less like the ancestral chordates than are cephalochordates or vertebrates.   At some time in the life cycle all possess a notochord, dorsal hollow nerve cord, pharyngeal gill slits, postanal tail, and endostyle but only the gill slits and endostyle are present in adults. Tunicates use the pharyngeal gill slits for suspension feeding. The larva is much more chordate-like than the adult and resembles a tadpole or fish, has all the chordate apomorphies, and is known as the tadpole larva. Metanephridia are absent and the coelom is reduced to a pericardial cavity and gonads. As in cephalochordates the gut is dominated by an enormous pharynx surrounded by a water-filled atrium but unlike cephalochordates, it is U-shaped with the mouth and anus anterior. Tunicates may be benthic or planktonic and solitary or colonial. All are marine.

            Tunicata is traditionally divided into Ascidiacea (the benthic sea squirts in three taxa; Aplousobranchia, Phlebobranchia, and Stolidobranchia), Thaliacea (the pelagic pyrosomes, doliolids, and salps), and Appendicularia (the pelagic larvaceans).   Recent molecular evidence and reevaluation of morphological evidence, however, suggests that Ascidiacea is paraphyletic and Tunicata should be reorganized into three different higher taxa (Fig 29-32).   In this reorganization Stolidobranchia would be one higher taxon.   Phlebobranchia plus Thaliacea would be the second taxon.   Aplousobranchia plus Appendicularia is the final tunicate taxon. For now, however, the traditional classification will be followed.

Ascidiacea C

            Ascidiacea is usually taken as representative of Tunicata, at least for the purposes of introductory laboratory exercises. Ascidians, or sea squirts, are sessile filter feeders that, as adults, bear little resemblance to their chordate relatives.   Ascidians have a living, external, cellular exoskeleton, or tunic, underlain by epidermis.   The tunic resembles connective tissue, except it isoutside the epidermis, and consists of cells, a secreted extracellular matrix, and ground substance.   Much of it is a cellulose-like polysaccharide.   In many ascidians blood vessels cross the epidermis to enter the tunic, a feature found in no other animal.

            The gut is U-shaped and both openings are anterior, with the anus dorsal to the mouth.   The gut, indeed the entire body, is dominated by an enormous pharynx whose wall is perforated by numerous tiny gill slits.   The pharynx is surrounded by a water-filled atrium into which the gill slits open and which itself opens to the sea.   It is both respiratory organ and filter-feeding device.  Water and food particles enter the pharynx and the water passes through the gill slits to the atrium and then out the siphon.   Food, entangled in mucus secreted by the endostyle, remains in the gut and passes posteriorly to be digested.  

            The hemal system includes a heart, vessels, and blood spaces in the connective tissue. The heart is enclosed in a pericardial cavity derived from the ancestral coelom.   The pattern of blood flow resembles that of cephalochordates and early vertebrates except that the heart reverses direction periodically and the blood thus flows in both directions through the system.  Ascidians have no structure recognizable as a kidney.  

            Ascidians are simultaneous hermaphrodites and the gonoducts open into the atrium.   Some ascidians are solitary and may be relatively large.   Others are colonial with tiny individual zooids in a common tunic.  

Enterogona O

            The gonads are in or beside the gut loop and the neural gland is below the cerebral ganglion.

Phlebobranchia sO

            Phlebobranchs have no postabdomen and may be solitary or colonial. The pharynx wall has raised longitudinal and transverse blood vessels but is not pleated. The inner surface of the pharynx bears projecting papillae that support the mucous feeding net.   Papillae on the inner wall of the pharynx help hold the mucous net in place. No postabdomen is present. The tail of the tadpole is vertical in all except Perophora.


Laboratory Specimens


            Ciona intestinalis, the sea vase, is one of the most widely distributed solitary ascidians.   It is found on the coasts of the Mediterranean Sea, Black Sea, northern Europe, the Arctic, Greenland, and the Atlantic coast of North America south to Massachusetts.   On the Pacific coast it extends south to southern California.   It has been introduced into major ports including many in the southern hemisphere. Because it is common, easy to dissect, achieves large size, and exemplifies the simplest, least specialized a simple ascidian body plan, it is frequently used in invertebrate zoology laboratories as a representative sea squirt.  

            Living specimens, if available are preferable to preserved for this exercise.   The dissection should be conducted in a small wax-bottom dissecting pan in isotonic magnesium chloride (or in water if preserved) with a dissecting microscope. Several hours of immersion of living specimens are required for complete relaxation in magnesium chloride. Comments about the color of structures apply only to living specimens.   Preserved material is usually uniformly gray or brown.   Commercial preserved specimens are often without their tunics.

            Specimens mounted in plastic may be available in your laboratory.   If so use one to preview the major features of urochordate anatomy before beginning your dissection.   Directions for use of plastic mounts will be found at the end of this exercise.

External Anatomy

            The narrow anterior end of the body is recognized by the two tubular siphons extending from it (Fig 1, 29-15A).   The inhalant buccal siphon, which is the anterior opening into the gut, is the larger and is terminal.   The exhalant atrial siphon, which is the opening from the atrium, is smaller and shorter. It is dorsal and a little posterior to the buccal siphon.   Small, red ocelli are located around the periphery of the two siphons but they are inconspicuous in preserved specimens.

            Determine the major body axes and find the plane of symmetry.   Be sure you know anterior-posterior, dorsal-ventral, and right-left.  

            The body is enclosed in a soft, gelatinous, translucent, greenish tunic, or exoskeleton, whose color is due to the presence of vanadium in blood corpuscles.   Blood vessels are visible in the tunic. (The tunic is often missing in preserved specimens.)     The posterior end of the tunic bears small projections, or holdfasts, which attach the animal to the substratum.   The name "Tunicata" refers to this exoskeleton.   The tunic is the outermost layer of the body wall.   The inner layer of the body wall is the mantle, consisting of a thin epidermis and underlying connective tissue and muscles.   Of these the muscle is all you will see.

            The body is divisible into a thorax, containing the pharynx and atrium, and a short abdomen with the posterior gut, heart, and gonads (Fig 1, 29-17C).   No postabdomen is present. The thorax is the long, narrow, anterior end whereas the abdomen is the short posterior end.  

Internal Anatomy

            Most of the study of the internal anatomy should be conducted with low power of the dissecting microscope.   The body wall, consisting of the tunic and mantle, is usually so transparent that many internal organs are visible through it.   Before opening the animal make a preliminary survey of the internal organs visible through the translucent body wall (Fig 1, 29-15A).   Five stronglongitudinal muscle bands are visible on each side of the body just inside the tunic.   A long white sperm duct can usually be seen through the dorsal body wall.   The large, horseshoe-shaped loop of the intestine is clearly visible through the body wall on the left side of the abdomen.   The pinkish brown (in life), pyriform ovary is located in the center of the horseshoe.   It is easily seen on the left, less so on the right.   The rectum, if it is full of brown fecal pellets, will be visible as a thick, dark, longitudinal column paralleling the dorsal margin and close to it.   The orangish stomach can be seen on the right side at the postero-dorsal extremity of the abdomen.   The tunic and mantle will be removed, in turn, to study the remaining internal anatomy.  

Tunic Removal

"     Skip this if you have a preserved specimen without its tunic. Be sure the animal is completely relaxed and cannot respond to physical stimuli and then remove the tunic. If the tunic is removed prematurely, the mantle muscles will contract and stay contracted, no matter how long they remain in magnesium chloride.   Without the tunic to oppose them there is nothing to restore the muscles to their extended resting position and they remain contracted.  

            To remove the tunic, gently push the viscera anteriorly to free a space in the posterior end of the sac made by the tunic.   With standard scissors, cut transversely across the now emptyposterior end of the tunic to open the space within.  

            Insert one blade of the scissors into this space and make a longitudinal incision anteriorly, into the thorax.   It does not matter where you make the incision as long as it extends from posterior to anterior.   Cut ONLY the tunic, not the animal within, but sever all connections between the tunic and the animal.   Most connections are at the siphons. Separate the body from the tunic and discard the tunic.   Usually the body slips out of the tunic easily but sometimes more effort is required to separate them.

            Before removing the mantle, look through it and identify some of the structures within.   You have already seen most of them but will now have a clearer view.   First, inspect the angle between the two siphons.   Here you will see a small, but conspicuous, white cerebral ganglion, or brain (Fig. 1, 29-15A, 29-23).   It is elongate and has nerves entering at each end.   It is very close to the surface and this is the best view you will get of it.   This ganglion and its nerves constitute the nervous system.   The wider, paler neural gland can be seen deep to it.   (The ganglion and neural gland under it are readily apparent in preserved specimens also.)

            The gut is the most conspicuous feature of the animal and it dominates the body of every ascidian (Fig 1, 29-15A).   Most of its important features can be seen now.   The buccal siphon opens into a gigantic pharynx that occupies most of the long, narrow thorax.   The pharynx is a large, white, gauzy bag filling the ventral 2/3 of the thorax.   It is absent in the dorsal thorax.   Theendostyle is a narrow, sinuous, white line that can be seen running along the ventral edge of the pharynx.  

Figure 1.   The solitary sea squirt, Ciona intestinalis, viewed from the right side.   The mantle muscles and pharyngeal gill slits (stigmata) have been omitted for clarity. Uro75La.gif

  Figure 1

            Posteriorly the pharynx empties into a short, narrow, white esophagus (Fig 1, 29-15A), which is difficult to see at present.   It is located at the postero-dorsal corner of the pharynx.   The esophagus leads to a large, swollen stomach on the right posterior side of the abdomen.   Its anterior walls are glandular and orange or brown, making it easy to recognize.   A thick intestine, usually dark with feces, exits the posterior end of the stomach and makes a horseshoe loop and then turns anteriorly as the rectum.   The loop of the intestine is most clearly seen on the left.  The rectum runs anteriorly through the thorax along the dorsal edge of the pharynx.   It usually contains brown fecal pellets.   It ends at the anus about halfway along the length of the thorax, in the atrium.   Formation of compact fecal pellets is important for filter feeding animals, especially those with the anus close to the filter intake, who wish to avoid fouling the filter with feces.

            The pharynx is surrounded on its lateral and dorsal sides by a water space called the atrium but you will not see it until later.   The atrium is filled with seawater and is continuous with the sea via the atrial siphon.  

            The ovary of these hermaphroditic animals is a conspicuous brownish pyriform (usually) organ located in the loop of the intestine on the left side.   It looks a little like tapioca pudding with its brown eggs clearly visible.   It empties into a large oviduct that runs beside the rectum to open into the atrium, at a position farther anterior than the anus.   You probably will not see the oviduct now unless it contains small brown, spherical eggs.  

            The testis is less obvious than the ovary and consists of diffuse whitish tissue on the otherwise dark surface of the stomach, intestine, and ovary.   It empties via a brilliant white (in life), very obvious sperm duct that runs beside the oviduct and rectum but is smaller than either.  

            If you have a living specimen remove it from magnesium chloride and place it in seawater at this time.   In seawater the heart may resume beating.

            The pericardium is an elongate sac and the heart is a tube lying inside it (Fig 1, 29-21A,B) on the posterior border of the pharynx on the right side of the stomach.   Using transmitted light, look carefully in the region anterior to the stomach and posterior to the pharynx.   You are looking for motion and, if the heart resumes beating you can see peristaltic waves move along it.   If the heart is not beating, you will probably not recognize it.   It, and the pericardium that encloses it, are transparent colorless membranes that are difficult to distinguish from other tissues unless the heart is beating.   Watch closely for several minutes for the heartbeat.   It is not dramatic and looks rather like a translucent curtain blowing gently in a breeze.

            The tube of the heart makes a hairpin loop and its two ends lie near each other.   A vessel exits each end to supply the body with blood and return blood to the heart.   Each of the two vessels serves both functions because the heart reverses the direction of its beat periodically.   Watch for reversal of the heartbeat.   One vessel runs in the direction of the pharynx, the other toward the stomach.   The former supplies (and drains) the dorsal pharynx and the test.   The latter serves the gut, ventral pharynx, and test.

            The epicardia are long blind pharyngeal diverticula lying beside the heart into which uric acid crystals are stored (Fig 29-22B).  


            The body wall inside the tunic is the mantle.   It consists of the epidermis, muscles and connective tissue but only the muscles are identifiable in gross dissection. The longitudinal muscles are the most conspicuous feature of the mantle.   In Ciona they are grouped in 10 longitudinal bands, five on each side.   Circular muscles are also present but are less well developed, not so obvious, and not in bundles.   They are best developed in the siphons.

Mantle Removal

"     Following instructions in the next paragraph remove the mantle from the abdominal region but do not attempt to remove it from the thorax. The pharynx is tightly attached to the mantle by numerous blood vessels that run between the two.   They can be separated only with difficulty and it is best to leave them together.   The mantle is more easily separated from the viscera of the abdomen and should be removed in this region.  

            To remove the mantle from the abdomen, place the squirt on its left side with its right side uppermost, facing you.   Slip one point of your fine scissors into the space between the mantle and the posterior edge of the abdomen and cut the mantle.   Extend the cut along the entire posterior, dorsal, and ventral edge of the abdomen.   The mantle will now slip off the abdomen.  Avoid damaging the tissues underlying the mantle, especially those in the loop of the gut. The delicate, transparent, membranous heart lies here.   Look once more for the heart if you did not see it earlier.   It, and the pericardium that encloses it, are transparent colorless tissues that are difficult to distinguish from other tissues unless the heart is beating.  

"   Place the animal on its left side (so the right side is facing you, as in Figure 1, 29-23.   Insert the point of a fine scissors into the buccal siphon and cut posteriorly on the right side, through the mantle and pharynx wall to the posterior end of the pharynx.   This will open the pharynx. The pharynx is a gauze-like bag extending ¾ of the length of the body.   Posterior to the pharynx the incision must be very shallow to avoid damage to the underlying heart, gut, and gonads.    Reflect and pin the mantle and pharynx wall.   Flush the mucus from the interior of the pharynx with a pipet or squirt bottly.

Buccal Cavity

            Look at the inside of the buccal siphon.   Anteriorly its lumen is the buccal cavity and the external opening is themouth (Fig 1).   There is a circumferential ring of delicate sensorybuccal tentacles around the siphon to sample incoming water.   Use higher power to examine the tentacles.   The slender circumferential peripharyngeal band encircles the anterior end of the pharynx just posterior to the ring of tentacles (Fig 1).  


            The ring of buccal tentacles marks the transition from buccal cavity to pharynx. The enormous pharynx occupies most of the space in the thorax. Its walls are thick and perforated by abundant tiny, oval gill slits (= stigmata) (Fig 29-26B).   The pharynx epithelium bears two types of cilia. Lateral cilia project into the gill slits, lie in the plane of the pharyngeal wall, and generate the feeding current.   The inner surface of the pharynx is covered by frontal cilia which are perpendicular to the pharyngeal wall and transport mucus over the walls.   Thousands of tiny, hookedpapillae extend from the walls into the pharyngeal lumen to support the mucus as it passes over the walls. The pharynx wall is not pleated longitudinally as it is in stolidobranchs such as Molgulaand the absence of these folds is a characteristic of Phlebobranchia (Fig 29-26B).   The pharynx has the double function of respiration and feeding.   It is a gill with large surface area and vascularized walls and is also a filter by which food particles are separated from water.  

Figure 2.   A dissected Ciona viewed from the left side.   The tunic and mantle have been removed.   The pharynx has been opened and deflected dorsally so it covers the rectum.   Uro76La.gif

Figure 2

            >1a. Look at the pharynx wall using the compound microscope.   Do this either with a prepared slide if one is available or with a wetmount you make yourself.   To make your own wetmount, remove a small piece of the wall of the pharynx and place it on a slide with a coverslip.   Be careful that you donnot include any of the mantle and that the piece of tissue is not folded.   

            Study the slide (whichever one you use) at 100X and 400X with the compound microscope.   The pharyngeal wall is penetrated by abundant oval gill slits.   The tissue between gill slits is almost entirely blood spaces containing circulating blood (Fig 29-26B).   The pharyngeal wall is, in fact, a network, or grid, of blood vessels perforated by gill slits.   Look for the lateral cilia lining the circumference of the gill slits.   If your animal is alive the cilia will probably be beating and blood corpuscles may be moving in the blood spaces.   The movement of these corpuscles is the easiest way to recognize the blood spaces.   Frontal cilia are usually not apparent.   Look for papillae projecting from the pharyngeal wall.<

            The endostyle of Ciona is a conspicuous, winding, white band on the ventral midline of the branchial pharynx (Fig 1, 2, 29-15A-C).   It begins at the junction of the two ventral ends of the peripharyngeal band and is a ciliated groove whose gland cells secrete a mucous net. It looks like a mountain road with switchbacks or a meandering river. Posteriorly the endostyle makes a small, tight, hairpin loop and then ends.   Its cilia move the mucous net onto the papillae of the pharyngeal walls.   The mucous net moves across the gill slits, propelled by frontal cilia and supported by papillae, where it traps food particles as they attempt to exit the pharynx through the gill slits (Fig 29-15D).   The mucus contains iodine and the endostyle is homologous to the vertebrate thyroid gland, which arises in the embryo from the floor of the pharynx.   The mesh of the mucous net diminishes the pore size of the filter and facilitates utilization of smaller particles.

            Along the dorsal midline of the pharyngeal lumen is a longitudinal row of long tentacle-like dorsal languets (Figs 1, 2, 3, 29-15B).   The languets gather the dorsally moving mucous sheet, roll it into a mucous string, and send the string posteriorly into the esophagus.   A mucous string may be present in the pharynx of your specimen. The row of languets is homologous to the dorsal lamina of Molgula and Ascidia and has the same function.   (The dorsal lamina is a continuous ribbon of tissue extending along the dorsal midline (Fig 29-15B). It is asymmetric with its free edge curved to the right.)


            Trace the row of languets (and the mucous string, if present) posteriorly to the end of the pharynx where it terminates at the small, puckered opening of the esophagus. The seophagus is a short tube that extends from the postero-dorsal corner of the pharynx to the much larger, wider stomach.


            The short esophagus turns dorsally to enter the large, oval stomach on the right posterior side of the abdomen.   The stomach walls are covered with orange glandular tissue which forms a simple pyloric gland.


            The tubular intestine, often dark with the feces it contains, exits the posterior dorsal end of the stomach, makes a sharp hairpin loop on the left side of the heart, and then extends anteriorly as the long, straight rectum (Figs 1, 2, 29-15A).   The rectum runs anteriorly through the thorax along the dorsal edge of the pharynx to the anus.   The rectum and anus are in the atrium.   When empty, its transparent walls are more difficult to see.


"     Unpin the specimen and turn it over so you can see the left side.   Use your fine scissors to open the atrium as you did the pharynx.   Insert the scissors into the atrial siphon this time and cut posteriorly along the dorsal midline.   Pin the atrial walls aside and study its interior.   Note the extent of the atrium and its spatial relationship with the pharynx.   The atrium extends to the posterior end of the pharynx and should be opened for its full length.   Find the anus at about the midpoint of the length of the atrium. It is at the end of the rectum, of course, and is surrounded by a circle of papillae.  

Reproductive System

            The gonads are on the left side between the stomach and pharynx (Fig 2, 29-15A).   In young individuals they may be small and obscure, in which case the heart must be moved aside to reveal them.   The ovary is a hollow, speckled, brown or white pear-shaped organ occupying most of the space in the loop of the intestine.   Its small brown eggs are delivered to the atrium by the transparent oviduct, which is difficult to see unless eggs are present.   The lumen of the oviduct is continuous with the lumen of the ovary.   The oviduct lies on the side of the rectum and is almost equal to it in diameter.   It exceeds it in length.   If it happens to have eggs in it, it will be easy to recognize.   Otherwise it will likely be missed.   Fortunately, eggs are stored in the oviduct and are usually present.  

            The large, but diffuse testis is less obvious than the ovary and consists of amorphous whitish tissue on the otherwise dark surface, both right and left, of the stomach, intestine, and ovary. It is connected by several small ductules to a conspicuous (sometimes), bright white vas deferens that runs across the anterior end of the ovary and then parallels the oviduct as it runs anteriorly. It empties into the atrium and if sperm are present, it will be bright white and easily seen. The vas deferens is much smaller in diameter than the oviduct but is more conspicuous because of its color. It is translucent white and may have orange tissues associated with it. The oviduct and vas deferens run on the ventral surface of the rectum but are much longer than the rectum and extend almost all the way to the atrial siphon.   

Neural Gland and Cerebral Ganglion

            The ovoid, bright white neural gland is on the dorsal midline of the anterior end of the pharynx (Fig 3, 29-23).   It is at the anterior end of the row of languets just posterior to the peripharyngeal band.   The neural gland is adjacent to the ventral side of the cerebral ganglion and appears to be a part of it, although it is not.   Immediately anterior to the neural gland is the transparent, rosebud-like dorsal tubercle.   The dorsal tubercle is a ciliated funnel leading from the pharynx lumen into the neural gland.   The neural gland moves seawater from the anterior pharynx into the blood.  It may be homologous to the vertebrate anterior pituitary, which arises in the embryo as an diverticulum of the roof of the buccal cavity.

            The cerebral ganglion is best seen by looking from the dorsal side through the mantle at the dorsal surface of the neural gland.   The cerebral ganglion lies on the dorsal surface of the neural gland and obscures your view of the gland from the dorsal side.   In life, the ganglion is bright white, whiter than the neural gland, and is an elongate rectangle with two nerves at each end.  It may have orange pigment on it.  

Excretory and Nervous Systems

            The epicardia are presumably accumulation kidneys that secrete uric acid crystals and store them indefinitely.   No renal bodies or renal sac are present and ascidians are completely lacking metanephridia. The nervous system consists of the cerebral ganglion and its nerves, which have already been discussed.  

Figure 3.   View of the dorsal midline of the anterior end of the pharynx viewed from inside the pharynx.   The pharynx has been opened and its walls deflected.   Uro77La.gif

  Figure 3

Tadpole Larva

            Examine a commercially prepared wholemount of a tadpole larva of unknown species.   Very little internal structure is discernable in most commercial slides but you can see the general shape of the larva and a few structures.    The tadpole consists of an ovoid anterior body with a long, slender, muscular, posterior tail.   In some preparations a thick, transparent, almost invisibletunic surrounds the body.   The notochord is usually visible in the center of the tail.   Within the body the cerebral vesicle (= sensory vesicle) should be apparent.   It contains at least one, sometimes two opaque spheres.   One is a statocyst for gravity detection and the other is an eyespot for photoreception.   The cerebral vesicle will become the cerebral ganglion of the adult. Two adhesive papillae are present at the extreme anterior end.   These will be used to attach to the substratum when the larva leaves the plankton and begins its sessile adult life.   The gut, with nonfunctional pharynx, endostyle, and intestine are usually an amorphous mass whose details cannot be recognized in these slides. Buccal and atrial siphons, although present, are closed and are not functional.

Plastic Embedded Specimens

            Stained specimens embedded in blocks of plastic are sometimes available from biological supply companies.   If available they can be used for a superficial study of ascidian anatomy or as a preview for a more detailed examination of a living or preserved specimen.   This part of the exercise is written to stand alone so it can be used instead of dissection for a brief study.  Treat the plastic blocks with care to avoid scratching the soft plastic.   Scratched plastic is difficult to see through.   Never place the plastic on the desktop or microscope stage without something soft such as a towel for it to rest on.

            Plastic mounted specimens will have either the right or left sides uppermost.   You can turn the block over to study the opposite side.   First you need to orient the organism.   Examine the block with 10X of the dissecting microscope. The narrow end is anterior and the expanded end is posterior.   The anterior end has the two tubular siphons (Fig 29-15A).   The ventral edge is best recognized by the conspicuous, long, dark line extending for most of the length of the animal.   This is the endostyle and it is ventral.   The side opposite it is dorsal   With this information determine which side is left and which is right.

            The body consists of a large thorax and a small abdomen.   The pharynx is in the thorax and the stomach and gonad in the abdomen.   Ciona is a solitary species so there are no connections with other individuals.   The body is enclosed in a transparent body wall which consists mostly of the tunic. In these specimens the tunic appears as a faintly visible, slightly stained outline around the more visible, more heavily stained viscera.   Blood vessels can be seen extending into the tunic.   The surface of the tunic bears small holdfasts used to attach the animal to hard substrata.

            Find the two tubular siphons at the anterior end of the specimen.   These are tubular passages for the flow of water into and out of the feeding apparatus.   The larger and more ventral of the two is the buccal siphon.   The atrial siphon is smaller and is dorsal.

            The buccal siphon opens into the gut and the atrial siphon opens from the atrium.   Water and food particles enter the buccal siphon and flow into the pharynx, whose walls are perforated by tiny gill slits (Fig 29-15A).   Water exits the pharynx by flowing through the gill slits into the surrounding atrium.   Food particles too large to pass through the slits must remain in the pharynx from which they are sent posteriorly into the digestive regions of the gut.   Water in the atrium exits via the atrial siphon.

            The mouth at the tip of the buccal siphon opens into a short buccal cavity inside the siphon.   The buccal cavity expands to form the enormous pharynx. The pharynx looks like a latticework of tissues and extends almost the entire length of the thorax. The lattice-like appearance is due to large transverse blood vessels and smaller longitudinal vessels in its wall.   The gill slits lie in the spaces between the blood vessels.  The gill slits themselves are difficult to see.   The endostyle, mentioned earlier, is a conspicuous black (stained) groove extending along the ventral margin of the pharynx.   It is a ciliated groove that secretes the mucus that traps food particles on the inside of the pharyngeal wall. A longitudinal row of dorsal languets extends along the dorsal margin of the pharynx.   These are small ciliated papillae that gather the mucous sheet and send it, with its trapped food particles, posteriorly to the esophagus and stomach (Fig 29-15B).  The dorsal languets are difficult to discern in these preparations but their position is faintly indicated by the slightly darker dorsal margin of the pharynx.

            With 20X look at the right side of the specimen.   Focus your attention on the dorsal posterior corner of the pharynx.   Here you will find the short tubular esophagus exiting the pharynx.  The esophagus is difficult to see from the left but is conspicuous on the right. The esophagus curves posteriorly and then curves sharply dorsally to expand into the bulbous stomach. The stomach will probably be darkly stained but its outline can be made out in the posterior thorax. It is posterior to the farthest extent of the pharynx.  

            The long tubular intestine exits the ventral end of the stomach and makes a large loop on the left side of the pharynx.   Turn your block over and look at the left side to see this loop. The loop takes the intestine from its ventral exit from the stomach to the dorsal side of the thorax.   Here it turns anteriorly and extends as a straight tube to the anus in the atrium on the dorsal side.  The intestine is often filled with sand, which makes it more visible, but parts of it may be empty.   The atrium is difficult to see, since it in only a space, with no tissues to absorb stain.   The relatively clear area along the dorsal midline between the anus and atrial siphon is part of the atrium.

            The gonads, ovary and testis, are located in the loop between the stomach and intestine on the left side. They appear as a dark mass in this loop.   You probably will not be able to distinguish between them.   Each has a gonoduct, oviduct and sperm duct respectively, that extends along the right side of the intestine to their gonopores in the atrium between the anus and atrial siphon.   You should be able to find at least one of these ducts. The sperm duct is usually easier to see than the oviduct.

            The small dark area on the anterior dorsal side between the buccal and atrial siphons is the cerebral ganglion (brain) and neural gland.   The heart lies to the right of the stomach cannot be discerned in these preparations.


            Alder J, Hancock A.   1905-07.   The British Tunicata.   Ray Society, London.

            Barrington, E. J. W .   1965.   The Biology of Hemichordata and Protochordata.   Oliver and

 Boyd,Edinburgh.   176p.

            Berrill NJ .   1950.   The Tunicata.   Ray Society, London.   354 p.

            Burighel P, Cloney RA.  1997. Urochordata: Ascidiacea, pp 221-347 in Harrison FW, Ruppert

EE (eds.) Microscopic anatomy of invertebrates. Wiley-Liss, New York. 537pp.

            Goodbody Ian.   1974.   The physiology of ascidians.   Adv. Mar. Biol.   12:2-148.

            Parker TJ, Haswell WA .   1951 .   A Textbook of Zoology, vol 1, 6th ed.   MacMillan, London.

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

            Van Name WG.   1945.   The North and South American ascidians.   Bull. Amer. Mus. Nat. Hist.

  84:1-476, pls 1-31.


Dissecting microscope

Isotonic magnesium chloride


Dissecting pan

Living or preserved Ciona

Tadpole larva wholemount slide

Compound microscope

# 1 stainless steel insect pins

Slides and coverslips