Invertebrate Anatomy OnLine
Bellamya japonica ©
Japanese Mystery Snail
24may2007
Copyright 2005 by
Richard Fox
Lander University
Preface
This is an exercise 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.
Systematics
Mollusca P, Eumollusca, Conchifera, Ganglionura, Rhacopoda, Gastropoda C, Prosobranchia sC, Caenogastropoda O, Mesogastropoda sO, Ampullarioidea SF, Viviparidae F (Fig 12-125)
Mollusca P
Mollusca, the second largest metazoan taxon, consists of Aplacophora, Polyplacophora, Monoplacophora, Gastropoda, Cephalopoda, Bivalvia, and Scaphopoda. The typical mollusc has a calcareous shell, muscular foot, head with mouth and sense organs, and a visceral mass containing most of the gut, the heart, gonads, and kidney. Dorsally the body wall is the mantle and a fold of this body wall forms and encloses that all important molluscan chamber, the mantle cavity. The mantle cavity is filled with water or air and in it are located the gill(s), anus, nephridiopore(s) and gonopore(s). The coelom is reduced to small spaces including the pericardial cavity containing the heart and the gonocoel containing the gonad.
The well-developed hemal system consists of the heart and vessels leading to a spacious hemocoel in which most of the viscera are located. The kidneys are large metanephridia. The central nervous system is cephalized and tetraneurous. There is a tendency to concentrate ganglia in or near the circumenteric nerve ring from which arise four major longitudinal nerve cords.
Molluscs may be either gonochoric or hermaphroditic. Spiral cleavage produces a veliger larva in many taxa unless it is suppressed in favor of direct development or another larva. Molluscs arose in the sea and most remain there but molluscs have also colonized freshwater and terrestrial habitats.
Eumollusca
Eumollusca, the sister taxon of Aplacophora, includes all molluscs other than aplacophorans. The eumolluscan gut has digestive ceca which are lacking in aplacophorans, the gut is coiled, and a complex radular musculature is present.
Conchifera
Conchifera, the sister taxon of Polyplacophora, includes all Recent molluscs other than aplacophorans and chitons. The conchiferan shell consists of an outer proteinaceous periostracum underlain by calcareous layers and is a single piece (although in some it may appear to be divided into two valves). The mantle margins are divided into three folds.
Ganglioneura
Most Recent molluscs are ganglioneurans, only the small taxa Aplacophora, Polyplacophora, and Monoplacophora are excluded. Neuron cell bodies are localized in ganglia.
Rhacopoda
The mantle cavity is posterior in the ancestor although it may be secondarily moved to an anterior position by torsion. This taxon includes gastropods and cephalopods.
Gastropoda C
Gastropoda is the largest molluscan taxon and is the sister group of Cephalopoda. Gastropods are united by descent from a torted ancestor although many exhibit various degrees of detorsion. Many are coiled and asymmetrical but the ancestor was probably symmetrical. Gastropods are relatively unspecialized molluscs known collectively as snails. The univalve shell, present in the ancestral gastropod and in the majority of Recent species, is reduced or lost in many representatives. The flat creeping foot was inherited from their eumolluscan ancestors but gastropods have developed a distinct head with an abundance of sophisticated sense organs. The originally posterior mantle cavity has become anterior as a consequence of torsion, although detorsion has reversed this condition in many. Gastropods were originally gonochoric and most remain so but many derived taxa are hermaphroditic. Most are marine but many taxa have invaded freshwater and the only terrestrial molluscs are gastropods. Most have a single gill, atrium, and nephridium but the most primitive representatives have two of each. Only one gonad, the right, is present. The ancestor probably had an operculum. The nervous system is streptoneurous (twisted by torsion).
Prosobranchia sC
Prosobranchia was once one of three great gastropod subclasses but it is no longer considered to be a monophyletic taxon, although the concept continues to be used as a pedagogical convenience. Prosobranchs are the gastropods most like the ancestral snails. They are torted and most have a shell and are coiled and asymmetrical. The mantle cavity is anterior. Most are gonochoric and most have an operculum. Most have only one gill in the mantle cavity but some primitive taxa have two. The right atrium is lost in most. Prosobranchs are specialized for life in marine benthic habitats although representatives are also found in freshwater and on land.
Caenogastropoda O
Caenogastropoda includes the two large and successful groups, mesogastropods and Neogastropoda. One gill, one nephridium, and one atrium are present. The gill is monopectinate, with filaments on only one side of the central axis. This new gill is less prone to fouling with sediment and silt and is probably largely responsible for the success of these snails as it allowed invasion of soft-bottom habitats.
Mesogastropoda sO
Although Mesogastropoda is no longer thought to be monophyletic it remains a useful pedagogic device and the concept is still widely employed. The mesogastropod osphradium is a simple ridge and the radula is taenioglossate, with seven teeth in each transverse row. Mesogastropods occur in marine, freshwater, and terrestrial habitats but most are marine.
Viviparidae F
These snails are viviparous and females are equipped with a uterus in which they gestate the eggs until they become juvenile snails. They release fully developed small snails, hence the scientific name Viviparidae and the common name "live-bearing snail".
Viviparids are large snails resembling their relatives the apple snails (Ampullariidae = Pilidae) but, in contrast with ampullariids, viviparids are not amphibious, do not have a lung, do not respire in air, and do not lay eggs. The male viviparid right tentacle is modified to serve as a penis. Ampullariids, which, like viviparids, are freshwater prosobranchs, have a lung and a gill, are amphibious and can respire in either air or water, lay calcareous eggs above the water line, are oviparous and do not gestate, and have identical right and left tentacles in both sexes. Mystery snails tend to have higher spires than apple snails.
Natural History
Viviparids are large freshwater snails known as “mystery snails” or “live bearing snails”. Mystery snails found in North America belong to several genera (Campeloma, Viviparus [ = Paludina] , Bellamya [ = Cipangopaludina] , Tulotoma, and Lioplax) in the family Viviparidae. Five genera and 19 species of viviparids occur in North American habitats. Of these most are native but some are introduced exotics. Bellamya is introduced, but Campeloma, Lioplax, and Tulotoma are endemic to North America. Viviparus is not endemic but is widespread and common in eastern North America where it is found in both lakes and rivers. Tulotoma is restricted to the Coosa River of Alabama.
Most viviparids are detritivores or microphagous browsers feeding on microalgae. Bellamya is a filter feeder and detritivore, but also browses on microalgae. In aquaria, and presumably in nature as well, Bellamya eats carrion when available. It does not feed on macrophytes, making it popular with aquarists and water gardeners. The spread of these snails across North America has probably been facilitated by these hobbyists. It sometimes appears in North American food markets catering to Asians. That a single gravid female is capable of founding a population helps explain their rapid dispersal.
With few exceptions viviparids are gonochoric. Females retain and gestate eggs in an enlarged uterine region of the pallial oviduct. Large shelled juveniles are released by the female at short intervals. Sexual dimorphism in Bellamya is slight, with males being smaller than females and with their right tentacle modified as an intromittent organ (penis).
Laboratory Specimens
Because of the effects of torsion and coiling on the molluscan body plan, gastropod anatomy can be confusing to beginning students and achieving an understanding of it is an objective that should be served by the laboratory experience. Unfortunately, the preserved snails traditionally used for this purpose are difficult and frustrating to dissect and often fail to improve the student's grasp of gastropod anatomy. Relaxed recently killed or living anesthetized snails, which have not been preserved, are much easier and more rewarding to dissect and are more likely to further an understanding of molluscan anatomy and the effects of torsion and coiling.
Viviparids are good representative mesogastropods, and of prosobranchs in general. Their value in the study of invertebrate zoology is enhanced by the fact that living specimens are often be available inexpensively and that most anatomical features can be observed with minimal dissection. For zoology courses taught at inland locations, viviparids are a convenient alternative to marine snails such as Littorina or Buccinum. Sometimes large specimens of various viviparid (or ampullariid, see the link to Pomacea) species can be purchased at reasonable cost from pet or garden stores. This trade has been curtailed somewhat by legislation prohibiting the sale of some species of freshwater snails because of the tendency for escaped individuals to found permanent invasive populations (of which Bellamya is an excellent example). Viviparids are usually called “mystery snails” or “live bearing snails” in the pet trade but the names and identifications used in many shops are not reliable. Ampullariids include the apple, ram’s horn, golden, and blue snails of the pet trade. This exercise should not be used with ampullariid snails. Use the Pomacea exercise instead.
Freshwater habitats in North America support several species of viviparid snails that can be used for the study of gastropod anatomy. The ideal choice would be one of the two large introduced viviparids, Bellamya japonica and B. chinensis, that are widespread and in some localities may be available in large numbers. They are introductions that should not be in North American habitats in the first place. Further they are not endangered or protected.
This exercise is written specifically for the introduced snail, Bellamya japonica (= Cipangopaludina japonica, the Japanese mystery snail or giant Asian snail), which differs only slightly from its congener Bellamya chinensis (= Cipangopaludina chinensis, = Viviparus malleata, the oriental mystery snail), also an exotic, naturalized species. The two are similar and some specialists consider them to be conspecific. Any other suitably large viviparid could also be used but most of these are natives.
The two Bellamya species were introduced around 1890 and are now widespread and apparently still spreading. They are harvested for food in Asia and also in North America to serve Asian markets. Bellamya achieves much larger sizes, up to 70 mm in shell height, than any of our native viviparids, of which the maximum is about 35 mm. Bellamya japonica is more high spired and angular than the similar B. chinensis but both species exhibit variability in shell shape.
Many anatomical features of Bellamya are visible on the surface of the animal (after removal from the shell, of course) and require little or no dissection. One great advantage is that almost all of the gut is visible on the surface making it relatively easy to trace its course through the body (a tedious and often unsuccessful task in most gastropods). When dissection becomes necessary it is usually minimal.
This exercise was prepared using specimens collected from the Saluda River immediately below the Buzzard’s Roost dam, which impounds the waters of Lake Greenwood in the South Carolina Piedmont. Bellamya was first reported from Lake Greenwood in 2005 by which time it was already abundant. Specimens 70 mm in shell height were been collected from Lake Greenwood during the preparation of this exercise.
Active Snails
> a. Place a living unanaesthetized Bellamya in a 12-cm culture dish of lake water on the stage of a dissecting microscope. If you take the time to do this you will see some features of the snail under natural, active conditions, which is more instructive, and more fun, than seeing them after being anesthetized or sacrificed.
Arrange the snail in the dish so its aperture (opening) and operculum are up, facing you, and then leave it completely alone without bumping, jarring, sliding or otherwise disturbing it. The slightest disturbance, including a shadow, will cause it to retract. Focus on the left side of the snail. At first the animal will be retracted completely within the shell whose opening, the aperture, will be plugged by a black, teardrop-shaped door, the operculum. While you are waiting you might look for epifaunal organisms growing on the outside of the shell.
When it feels safe, the snail will move the operculum away from the aperture and extend its foot and head. You will see the dark slate gray integument, speckled with gold pigment spots. The gold color fades after death.
A snail body consists of foot, head, and visceral mass. Of these, the head and foot are visible but the visceral mass is completely enclosed in the shell which it never leaves. At first the large muscular foot will be folded on itself so you cannot see its true shape. The operculum is attached to the foot. The small head sits atop (dorsal to) the foot at the anterior end. You will probably be looking at its left side and if so you will see the left cephalic tentacle with a small eye near its base. The eye is on a short ocular peduncle fused with the lateral edge of the tentacle. To the right of the left tentacle, and thus on the midline of the head, is the short tubular snout with the mouth in the center of its free distal tip. You will not see the mouth unless the animal happens to point the snout at you while you are watching. A right tentacle is present to the right of the snout but you probably won’t see it if you are looking at the left side. Don’t try to reposition the snail or it will retract into the shell.
Above the head you will see the large arch formed by the edge of the shell. Lining this arch is a ridge of tissue known as the mantle collar. It is the edge of the mantle skirt. The edge of the shell arches over a large opening into the spacious cavelike mantle cavity. The mantle skirt is the roof of the mantle cavity. Focus through the opening on the numerous, fine, parallel filaments of the gill hanging from the roof.
Look at the left side of the inconspicuous neck connecting the head with the foot. To the left of the left tentacle you will see a thin earlike flange of tissue, the left nuchal lobe (nuchal = neck). The snail can roll this lobe into a short tube and use it as an inhalant siphon to bring water into the mantle cavity to flow over the gill. A similar, but larger, right nuchal lobe (exhalant siphon) is present to the right of the right tentacle but you probably won’t see it from your viewpoint.
The snail will attempt to extend the foot and right itself with the sole of the foot down, against the bottom of the dish. If it accomplishes this, you will get to see the foot in its normal, extended condition with the operculum attached to the top of its posterior end. The anterior end is under the head. When the snail contracts it will fold the foot in half with the anterior and posterior ends of the sole facing each other and then withdraw it into the aperture. This leaves the operculum on the outside, blocking the aperture. <
> b. If active undisturbed snails are maintained in an aquarium in your laboratory, spend a little time observing them without disturbing them. In these conditions you can see the features observed earlier under more natural conditions. Viewing will be best if you can find a snail on the front wall of the aquarium so you can look through the glass to see the animal (Fig 1, 2).
Under these conditions the foot is seen fully extended and in motion. Its ventral surface is the sole, which you can see is a pale, broad, flat oval attached to the glass. The operculum is on the posterior dorsal surface of the foot and may be out of your line of sight. The head is dorsal to the anterior end of the foot. The large cylindrical snout protrudes anteriorly from the midline of the head. The slitlike mouth is in the center of the free, distal tip of the snout. You will probably see the snail apply the tip of the snout to the substratum as it feeds on microalgae growing on the glass. If the snail is on the front glass of the aquarium, you may be able to see the rasplike radula protrude from the mouth and scrape against the glass to remove algae.
Figure 1. Bellamya in ventral view on the glass wall of an aquarium. Gastrop77L.gif
A conspicuous cephalic tentacle can be seen on each side of the snout. In males the right tentacle is thicker than the left and is curved at the tip. It functions as a penis and its presence is the only reliable external means of determining the sex of an individual. Watch as the animal uses the tentacles to investigate the environment ahead of its motion. A small black eye is situated laterally near the base of each tentacle at the tip of an ocular peduncle. The head connects with the foot and visceral mass via a short neck.
If your viewing angle permits, you may see, above the head, the opening into the mantle cavity. The shell, of course, encloses the visceral mass and is dorsal to the foot and posterior to the head (Fig 4).
An inhalant siphon is present on the left side of the neck and an exhalant siphon on the right. Both are formed when the animal rolls the left and right nuchal lobes, into tubes. The exhalant siphon, on the right, is much larger and usually the easier to see (Fig 1). In an undisturbed active animal it is a large diameter tube on the right of the snout. Be sure you understand how the snail can make a tube out of a flat nuchal lobe. You could do the same thing with a piece of paper. The inhalant siphon is to the left of the snout. A short ridge of tissue encloses the cephalic food groove and can be seen extending from the base of the right tentacle to a position under the snout (Fig 1). The groove transfers food from a food collection device in the mantle cavity to the mouth. <
Figure 2. A male Bellamya in ventral view from the left as it crawls on the glass wall of an aquarium. Gastrop83L.gif
> c. If adult Bellamya have been maintained in a laboratory aquarium for a few days, juvenile snails may be present (Fig 3). If so, place one of the small snails in an 8-cm culture dish of lake water and observe it with the dissecting microscope. Juveniles are not as shy as adults and you won’t have to wait long before it emerges from its shell. Start out with the snail upside down with its aperture facing up, toward you. Watch as the snail extends its foot and head from theshell. Note the sequence of events involving the operculum, foot, and head. It is the reverse of the sequence employed when the snail withdraws.
At first the animal is completely withdrawn with the operculum covering the aperture. The operculum moves aside as the folded foot, with the operculum stuck to the dorsal side of its posterior half, emerges first. Then the head emerges and the foot unfolds and attaches to the substratum. As the head emerges you will get a quick look at the snout, left cephalic tentacle, left nuchal lobe, mantle, and mantle cavity but as soon as the anterior foot contacts the substratum it will pull the shell around onto its life position above the foot so you can no longer see much of the head. Turn the snail back over so it has to emerge again to give you another look. Repeat this as often as necessary to see everything you want to see. Try to get a look into the mantle cavity to see the gill on its roof.
Look at the shell. It is fragile, compared with that of adults, and consists of a large diameter body whorl at its base with progressively small and smaller whorls spiraling above it (Fig 4). It probably has four whorls, or turns around its central axis. In these young snails the protoconch is still present and undamaged. The protoconch is the oldest and smallest whorl of the shell and sits at the apex. It is the shell of the larval or late embryo snail and because of its delicacy it is rarely present in adult snails.
The long, dark, tubular rectum may be visible through the translucent shell. The rectum is a long spiral band just under the shell of the body whorl. It is usually darker than the surrounding tissues. <
Figure 3. Two views of a 7 mm juvenile snail removed from the uterus of a 55 mm female. Gastrop92L.gif
Relaxation and Anesthetization
Fresh specimens for dissection should be relaxed/anesthetized so that the foot and head are extended from the aperture and then extracted from the shell. The specimen may be dead or alive and anesthetized. Molluscs are notoriously difficult to relax in an extended, relaxed condition. Under almost any adverse conditions, including immersion in anesthetic, the snail withdraws into its shell and waits for conditions to improve. It is possible to dissect unrelaxed snails but relaxed specimens are better subjects. Unrelaxed snails will have the foot folded tightly on itself and the head will be retracted into the mantle cavity. Relaxed specimens usually are only partly relaxed and the foot will be loosely folded. Four methods for preparing snails are suggested below but none is ideal. Instructions for extracting the snail from its shell appear later, in the Extraction section.
Method 1. Kill the snail by freezing it slowly in a 12-cm culture dish of tapwater. The snail must not experience any disturbance while freezing. The snail will usually freeze in a partially extended, dissectible condition. Freezing requires several hours and should be initiated by the teaching staff the day prior to the lab. Each snail should be frozen in a separate container of water so that the snails do not disturb each other. The snails should be placed upside down, with the aperture up, in their dishes. This technique results in partially relaxed, dead specimens but may present logistic difficulties for the teaching staff.
Method 2. Remove an unrelaxed snail from its shell (see Extraction section below). The snail can then be studied unrelaxed and unanesthetized or it can be immersed in 7 % non-denatured ethanol (made up in lake water) for relaxation and anesthetization. Study of the snail can begin immediately and the snail will die after about four hours immersion. If ciliary currents are to be studied the snail should not be placed in alcohol. This method minimizes demands on the teaching staff.
Method 3. A technique that results in living partially relaxed specimens is to place the snail in a dish or dissecting pan of 7% non-denatured ethanol with the foot extended and the aperture open. The anesthetic must have access to soft tissues which means the animal cannot be allowed to close the aperture. Surprise an extended animal in the aquarium and, before it can withdraw, prop the operculum open with something, such as a piece of a wooden applicator stick placed crossways between the foot and shell so the operculum cannot close and the anesthetic can reach the soft tissues. Hold this bridle in place with rubber bands and immerse the snail in alcohol. This is also a lengthy process requiring several hours and should be initiated early by the teaching staff. It is labor intensive.
Method 4. A snail can be placed in a culture dish completely filled with boiled (and thus anoxic), then cooled, water and covered with another dish so the snail has no access to oxygen. The dish must remain undisturbed during the process. The snail will eventually succumb in a partially extended condition and can be transferred to 7 % ethanol. The snail will be alive and partly relaxed.
Anatomy
Shell
Examine the external features of an empty or occupied shell. The snail inhabits a shell composed of calcium salts and protein. (The thickness of the shell varies with species and environmental conditions.) The gastropod shell is a cone wrapped around a central axis to form a spiral consisting of a series of revolutions, or whorls, around the axis (Fig 4, 12-27A,B). The shell of Bellamya is inflated (with swollen sides). It consists of one large body whorl, which contains most of the soft parts of the animal, and several smaller whorls that form a spire extending above the body whorl.
The body whorl opens to the exterior via a large opening, the aperture, which is on the right side of the central axis of the shell when the spire is pointing up and the aperture is facing you. Snails with the aperture in this location are said to be dextral, or right-handed (Fig 4). If the aperture were on the left, the snail would be sinistral, or left-handed. Bellamya, like most snails, is dextral. The aperture of most viviparids is less than half the height of the shell.
The whorls are wound spirally around a central axis called the columella (Fig 4, 12-27A). This axis is in the center of the spiral of whorls and consequently is not visible in an intact shell. The shell is covered on the outside by a thin greenish brown, proteinaceous periostracum but the remainder of the shell is calcium carbonate deposited on a protein matrix. The periostracum, operculum, and matrix are made of the protein conchiolin. Under magnification, use a scalpel to scrape a small spot on the shell to remove the periostracum and reveal the underlying white calcareous portion of the shell.
The operculum is a thin teardrop-shaped disk of flexible conchiolin that forms a door to close the aperture. It is attached to the foot of the snail as you will soon see. In viviparids, the growth lines of the operculum form concentric rings around a "center", or nucleus.
Figure 4A. The shell of Bellamya. A window has been cut in the body whorl to reveal the columella. Gastrop175L.gif
4B. The operculum. Gastrop74L.gif
Extraction
The currently visible soft anatomy of your specimen (Fig 1, 2, 2-16C, D) includes the large, flat-soled foot which, if your specimen is incompletely relaxed, is probably partly folded on itself along a transverse crease. The head is dorsal to the anterior end of the foot. The visceral mass extends from the dorsum of the foot up into the whorls of the shell and is not visible in intact specimens. The snail must be removed from its shell before soft anatomy can be studied.
A snail is attached to its shell by a single muscle which must be detached in order to remove the animal without damage. This attachment is the large, white right columellar muscle which extends from the posterior dorsal area of the foot to the columella, or central axis, of the shell. Its origin on the columella cannot be accessed from the intact aperture so you will have to crack and remove most of the shell to get to it.
" Remove the snail from its shell as follows. Place the body whorl in a small C-clamp (or small vise) and crack the shell with pressure applied slowly. Do not attempt to do this with a hammer. The C-clamp permits controlled application of pressure that is not possible with a hammer. Once the shell cracks, immediately release the pressure and use heavy forceps to remove the broken pieces of shell. Reposition the remaining shell in the clamp and apply pressure again until something else breaks. Never apply pressure to anything that yields, either broken shell or soft tissue. Remove the broken pieces of shell as before.
Continue this process until you have removed most of the body whorl and can see the white columella in the core of the shell (Fig 4, 12-27A). Find the large white columellar muscle extending from the foot to the columella. The columellar muscle has a broad oval origin on the columella. Use a scalpel to scrape, not cut, the muscle away from the columella. (Freezing sometimes causes the muscle to detach from the columella spontaneously making it unnecessary to scrape it loose.) Now gently try to unwind the snail out of the remaining shell. Do not use force to remove the snail. If the snail does not slide out readily, resume cracking. Check for an intact part of the muscle attachment you missed. Do not pull on the snail to remove it. It should yield to a very gentle tug. More than that will tear the tissue of the visceral mass and make things difficult for you later.
Examine the extracted animal. Copious amounts of opaque, elastic, white mucus will be present and should be removed as it appears. Most of the mucus will be in the mantle cavity, on the head, or the sole of the foot, i.e. the surfaces exposed to the environment in a normal snail. Use a disposable plastic Pasteur pipet to wash the mucus away. You can also take the snail to a sink and run a gentle stream of water over the sole of the foot, head, and into the mantle cavity to rinse away the mucus. Whatever method you use, be gentle with your specimen. Without their shells molluscs are delicate and easily damaged. Continue to remove mucus as necessary throughout the dissection.
Soft Anatomy
Place the extracted snail in a small dissecting pan and cover it with water or7% ethanol. Place it on the stage of a dissecting microscope.
Make a preliminary survey of the major features of your extracted snail (Fig 5, 11, 12-14). The broad, normally flat, muscular foot is ventral to everything else but it is probably partly or completely folded so the anterior half faces the posterior half on either side of a transverse fold in the middle of the foot. The foot is tough and rubbery. The head, with two lateral tentacles and a median snout, is located above the anterior end of the foot. In unrelaxed specimens it will be wholly or partially withdrawn and hidden under the mantle. The integument of the head is variously pigmented in different species but that of Bellamya is black or lead gray with striking gold chromatophores (in life).
The remainder of the body is the visceral mass. It is coiled and sits on the dorsal side of the foot. All of the body posterior to the head and dorsal to the foot is visceral mass. It is tapered and spiraled to fit in the upper whorls of the shell. It contains most of the snail's organ systems, which in this species, are conveniently visible through the body wall (Fig 5, 18, 11). The most obvious of these organs are the dark tubular rectum, conspicuous on the surface of the second and body whorls, and the large yellowish-brown digestive ceca occupying, to the exclusion of everything else, the uppermost whorls and sharing the intermediate whorls with other systems. The body wall covering the visceral mass is thin and is usually translucent so organs under it can be seen without dissection. In some specimens it may be pigmented making it more difficult to see the internal organs. If so, the black pigment can usually be scraped or picked away to leave the body wall transparent.
Figure 5. Dorsal view of a male Bellamya with the shell removed. The animal is partly relaxed and the foot is folded on itself. Most of the head is retracted into the mantle cavity. Gastrop71L.gif
The mantle (= pallium) is the dorsal body wall of the visceral mass (Fig 1, 12-14A) invaginated to form a double layer of itself over a deep dorsal pocket. The large pocket thus formed is the mantle cavity, which in life is filled with water (Fig 12-53). It is an important feature of mollusc anatomy. The double sheet of mantle is the mantle skirt. The mantle skirt is the roof of the mantle cavity and is a double layer of the body wall. The floor of the mantle cavity is the non-doubled, non-folded dorsal body wall. In fresh specimens the mantle skirt is pale gray or white, in sharp contrast with the black and gold of the head and dorsal foot.
You are now ready for a more detailed study of gastropod anatomy.
Foot
The foot is a large mass of muscle making up the ventral part of the snail (Fig 1). Its smooth (when extended), oval, ventral surface is the sole and is the part that contacts the substratum when the snail is crawling. It is equipped with abundant mucous gland cells whose secretions lubricate the substratum over which the snail crawls. If you are working with a fresh specimen you have no doubt encountered the copious mucus secreted by these cells.
When retracted, the foot folds transversely across its middle to fit into the available space inside the body whorl of the shell. The foot of your specimen is likely to be folded in this manner (Fig 5). If it is, the anterior and posterior halves of the sole will be facing each other. The horny proteinaceous operculum is a thin brown disc attached to the dorsal surface of the posterior half of the foot (Fig 4B). It is a door to close the shell aperture when the foot and head are retracted.
Head
The head of your specimen may be withdrawn entirely or partially under the mantle skirt. If it is, push the skirt out of the way to expose it. It is connected to the anterior visceral mass by a short neck (Fig 12-14A). The head and foot are unaffected by torsion and coiling and exhibit bilateral symmetry. Find the right and left sides of the head and foot. The visceral mass, on the other hand, is asymmetric, being both torted and coiled.
The anterior end of the viviparid head bears a cylindrical median snout with the mouth at tip (Fig 5, 1). The mouth is flanked by a pair of fleshy lips (Fig 2, 13). The snout is not a retractable proboscis such as is found in some snails. Two long cephalic tentacles arise at the base of the snout, one on each side. The tentacles may be partially contracted and not as long and slender as they would be in life. Laterally, near its base, each tentacle bears a small but conspicuous black eye on a short ocular peduncle (Fig 1). The ocular peduncle is fused with the tentacle.
The right tentacle of male viviparids is modified to serve as a penis, or intromittent organ. Relaxed, it is thicker and is tip is slightly blunter than that of the left. The tip is held in a curve like a shepherd's hook. If you think your specimen is a male, examine the tip with higher magnification (about 20X) to find the tiny distal male gonopore. With fine forceps gently squeeze the penis while watching the tip. A thin stream of semen will be extruded from the opening (of mature specimens) to verify its presence.
Lateral to the tentacles, the two sides of the neck each bear a thin flap of tissue, called a nuchal lobe (nuch = neck) that can be rolled into an extensible tube, or siphon. Find the left and right nuchal lobes (= labial organs, nuchal crests, epipodia). When rolled, the left lobe forms the tubular inhalant siphon to bring a respiratory water current into the left side of the mantle cavity where it flows over the gill. The equivalent on the right is the exhalant siphon through which the respiratory current exits the mantle cavity. These lobes are highly extensible but in contracted specimens they are small. In dead or relaxed specimens the lobes will not be rolled to form tubes. The right lobe is much larger than the left and when rolled forms a larger, more noticeable tube. Between the right nuchal lobe and the snout is an additional low, thin ridge of tissue that forms a short tube. It curves around the outside of the base of the right tentacle and ends near the mouth. This is the cephalic food groove and it is a part of a filterfeeding apparatus which will be discussed later.
Mantle Cavity
A double fold of the mantle, known as the mantle skirt, arches over the anterior dorsal visceral mass. The deep, triangular pocket below the skirt is the mantle cavity (Fig 1, 5, 12-53). The mantle cavity covers the dorsal surface of the body whorl but extends, as the narrowing apex of an elongate triangle, around the entire revolution of the body whorl. The base of the triangle is anterior, above the head. Gently slip a blunt probe into the mantle cavity and plumb its depths.
The mantle skirt (= mantle fold, mantle flap) is the roof of the mantle cavity. The mantle cavity opens to the exterior via a large opening above the neck and under the skirt. The floor of the mantle cavity is the dorsal body wall of the anterior visceral mass.
The free anterior edge of the mantle skirt encircles the dorsal body as a opaque mantle collar, located immediately posterior to the head and surrounding the opening into the mantle cavity. It is thicker than the rest of the skirt and consequently is opaque. In an intact snail the mantle collar lines the free edge of the shell around the aperture, as you observed earlier (Fig 1).The mantle collar fits against the edge of the shell around the aperture and the epithelium of its anterior border secretes the growing edge of the shell. Growth of the shell is dependent on the secretory epithelium of the free mantle collar. This secretory anterior edge of the mantle collar is distinguishable as the shell gland (not to be confused with the female's egg shell gland).
Several organs are associated with the roof of the mantle cavity and many important features of snail anatomy can be seen here prior to dissection. The mantle cavity of viviparids, like that of most prosobranchs, is a single undivided space containing, among other organs, a gill. This contrasts with that of the related ampullariids in which the mantle cavity is divided into a gill chamber (= branchial chamber) for aquatic respiration and a lung for aerial respiration. The mantle cavity of pulmonates, such as Helix and Limax, is an air-filled lung and no gill is present.
Orient the snail in the dissecting pan with the foot and operculum down, against the wax and the dorsal surface of the head up. The anterior end should be toward you and the posterior end extending away from you. Anchor the snail in place by forcing two stout # 7 stainless steel insect pins through the operculum to hold it flat against the wax of the dissecting pan. Be prepared to remove these pins when necessary to reorient the snail.
Begin your study of the mantle cavity with an examination of the outer surface of its roof. With forceps stretch the roof taut so its surface is more or less flat. Scrape any black pigment off of the surface. Many features are visible externally as oblique bands extending right to left across the roof (Fig 5). The left side of the roof is the same in both sexes but the right side differs. The structures in the roof of the mantle cavity are, in order from left to right, the efferent branchial vessel (narrow, white, transparent tube), osphradium (short, narrow, white line but barely visible externally), gill axis (broad white band), afferent branchial vessel (narrow, yellowish, transparent tube), open area over the free ends of the gill filaments (wide, white, translucent area), hypobranchial gland (narrow, yellowish-white band), rectum (dark, conspicuous, band), and ureter (translucent inconspicuous band, and uterus (females only, very large, transparent, with small snails inside). In females the ureter is difficult to distinguish from the adjacent uterus. In males the ureter is easily seen and the tip of the testis can be seen to the right of the upper ureter (Fig 5, 18). Although visible over most of the mantle skirt, these organs cannot be seen through the thicker mantle collar along the anterior border of the skirt.
" Open the mantle cavity using fine scissors to make a shallow oblique cut across the roof of the cavity along the right margin of the gill axis. This cut should be very shallow to avoid cutting the gill filaments which extend from the gill axis to the right, towards the hypobranchial gland. Deflect the right and left sides of the roof and look at the inner surface of the left side.
The gill (= ctenidium) is on the left side of the roof of the mantle cavity (Fig 5, 12-14B). It consists of a wide longitudinal central axis from which numerous ciliated filaments extend to the right. The central axis is the band of tissue between the two branchial blood vessels. The filaments are unusually long and slender and attach to the mantle roof along the central axis from which their free ends extend far to the right in the mantle cavity. The filaments are not held together by tissue or ciliary interfilamentary junctions and easily become separated from each other and appear disorganized when disturbed.
> d. With fine scissors remove a few filaments by cutting across their bases. Transfer them to a slide and make a wetmount for examination with the compound microscope. A longitudinal muscle is easily seen extending the length of the gill. Estimate the ratio of filament length to the width of the base. That of Viviparus viviparus is 26:1) <
This is a monopectinate gill with only one row of filaments arising from the axis. All mesogastropods and neogastropods have monopectinate gills. The gill extends all the way to the posterior end of the mantle cavity and you cannot see all of it in dorsal view. The left gill is the only one present in almost all prosobranchs, including viviparids.
Afferent and efferent branchial blood vessels extend along the two long margins of the gill (Fig 5, 11, 12-52). The afferent vessel from the body (kidney) is on the right, the efferent to the heart is on the left side of the gill axis. These deliver unoxygenated blood to the gill (afferent vessel) and drain oxygenated (efferent vessel) blood from it. The efferent vessel empties into the atrium of the heart.
The narrow, sometimes inconspicuous, sensory osphradium is attached to the roof of the mantle cavity along the base of the inhalant (left) side of the gill and efferent branchial vessel. In most prosobranchs the osphradium is obvious and resembles a miniature bipectinate gill with a central, longitudinal axis from which arise two rows of filaments. The viviparid osphradium has no filaments and does not resemble the gill at all. That of Bellamya is a short yellowish-white line extending a few millimeters along the anterior end of the gill and efferent branchial vessel.
The respiratory current enters the mantle cavity trough the inhalant siphon (formed of the enrolled left nuchal lobe) on the left and crosses the osphradium. It then passes posteriorly in the left side of the mantle cavity and flows across and between the gill filaments to the right side of the mantle cavity. From here it moves anteriorly to pass over the anus and urinary pore before exiting through the exhalant siphon (formed of the right nuchal lobe).
The narrow, elongate hypobranchial gland is slender yellowish-white band on the left edge of the rectum (Fig 5, 11). Most of the mucus remaining in the mantle cavity of your specimen is probably still attached to this gland, from which it was secreted. Other mucus-secreting gland cells are scattered over the floor of the mantle cavity.
The rectum is a large tube lying immediately to the right of the hypobranchial gland (Fig 5, 11, 12-14B). It emerges from the left side of the visceral mass and runs obliquely across the right side of the roof of the mantle cavity. When it contains feces it is dark and easily seen and recognized. The empty rectum of a starved animal is less obvious. When feces are present, the rectum is a dark gray, brown, or black tube, the color ultimately depending on diet. The extreme anterior end of the rectum is not attached to the mantle and forms a short, free-hanging rectal papilla with the anus at its tip. This papilla can be seen on the right just inside the mantle cavity. The anus lies in the outgoing stream of water exiting the mantle cavity on the right through the exhalant siphon.
Find the thin-walled, transparent, large-diameter, tubular ureter (= urinary sinus) paralleling and adjacent to the rectum on the right side of the rectum. It is easier to see in males than in females, in which it is difficult to distinguish from the similarly thin-walled membranous uterus. The ureter runs from the nephridiopore of the kidney, in the far posterior end of the mantle cavity, to a large urinary pore near the anterior border of the right side of the mantle cavity. The urinary pore in males is to the left of the anal papilla and a little posterior to it. In females it is between the bases of the rectal papilla and the much larger, but similar, vagina (discussed below). Insert the tip of a standard, bent teasing needle into the pore and push it gently up the ureter for a short distance. This is the best way to find the ureter in females. Flow through the urinary pore is regulated by a sphincter muscle. Both anus and urinary pore release wastes into the exhalant current.
The ureter is not part of the ancestral metanephridium. Instead it is a device derived from the mantle epithelium to transport urine from the nephridiopore in the posterior mantle cavity to the exhalant siphon in the anterior cavity. It ends at the urinary pore which is not on a papilla. Being a freshwater animal inhabiting a hyposmotic environment, Bellamya produces copious dilute urine requiring a large diameter duct for its transport.
In females the uterus is a conspicuous feature of the right side of the mantle cavity (Fig 11). It is a thin-walled, large-diameter, transparent tube lying to the right of the ureter and rectum. It is typically filled with several large juvenile snails and many smaller embryos. It extends along the right side of the mantle cavity to a position near the mantle collar. Here it narrows to become the muscular tubular vagina which is hangs from the mantle roof, as does the rectal papilla. The vagina is the larger of the two. The female gonopore is at the tip of the vagina. The vagina regulates the exit of juveniles from the ureter and receives the penis during copulation.
Look now at the floor of the mantle cavity. The floor is the dorsal body wall covering the anterior visceral mass and cephalic hemocoel immediately posterior to the head.
The topography of the floor is dominated by a feeding groove complex, which extends obliquely the length of the mantle cavity. It consists of two ridges with a groove between them. This complex begins in the posteriormost apex of the mantle cavity and extends anteriorly to the right side of the cavity to end between the right nuchal lobe and the right tentacle. Several functions have been attributed to this complex including channeling the flow of water through the mantle cavity, protecting the gill filaments from crowding by developing embryos, and filter feeding. It is similar, and presumably homologous to the epitaenium (epi = upon, taen = ribbon) of the amphibious ampullariid snails. In ampullariids the epitaenium channels water to the gill during aquatic respiration and to the lung during aerial respiration. Viviparids, however, are not amphibious and have no lung, so it seems unlikely that its chief function lies in channeling water to or away from the gill.
It is probably a feeding mechanism for collecting and transporting food particles to the mouth and it is described here in that context. It consists of a large sharp ridge of tissue on the left, which is the epitaenium (= branchial fold), and a much smaller, low, rounded secondary fold paralleling it on its right (Fig 6). Mucous gland cells are concentrated in a line along the base of the secondary fold. Between the two ridges is a shallow broad food groove lined with a ciliated epithelium. The thin crest of the epitaenium curves to the right and overhangs the food groove (Fig 6). It borders the food groove on the left. The groove is bordered on the right by the secondary fold.
Figure 6. Cross section of the epitaenium and food groove of a male. Gastrop72L.gif
In males this entire apparatus extends along the top of the large prostate gland for much of its length. The prostate extends about two thirds of the length of the mantle cavity but the epitaenium runs for its entire length. In females the epitaenium occupies the same position and is large and conspicuous, even though it is not on top of a prostate gland.
Anteriorly the epitaenium ends on the right side of the base of the right tentacle. The secondary fold and food groove, however, continue on as the cephalic food groove, exiting the mantle cavity to curve ventrally around the right tentacle to end ventral to the snout (Fig 1). The secondary fold is the right wall of the cephalic groove and the tentacle base is its left. The cephalic food channel is clearly visible as a curved ridge in ventral view of living snails crawling on the walls of aquaria.
The feeding apparatus for Viviparus viviparus has been described in the literature and is anatomically similar to that of Bellamya japonica. In V. viviparus the equipment is used for filter feeding and it seems cleat that is its purpose in Bellamya also. In Viviparus suspended food particles enter the mantle cavity with the respiratory current. The particles are caught on the gill filaments, mixed with mucus and moved by frontal cilia to the free tips of the filaments, where they drop off onto the ciliated floor of the mantle cavity. The filaments extend from the central axis to the right so their tips are located over the food groove on the floor of the mantle cavity (Fig 6). Particles that fall to the right of the secondary groove are caught in ciliary currents and moved to the left, over the secondary fold and into the food groove whereas particles that fall on or to the left of the epitaenium are moved to the right over the epitaenium and into the groove. Once in the groove, particles and mucus are moved anteriorly by its ciliated epithelium. A mucous string with entrained food particles moves anteriorly into the cephalic food groove curving around the right tentacle to the base of the snout where it accumulates. The snout periodically bends and the mouth ingests the mucous mass. The feeding apparatus has no provisions for sorting.
Figure 7. View of the area between the body and second whorls showing the apex of the opened mantle cavity and nearby organs. The animal has been rolled a little to the left, in comparison with Fig 5, to bring the right side into view. The pericardial cavity has been opened. Gastrop73L.gif
> e. To demonstrate particle transport and formation of the mucous string by the feeding apparatus you or your instructor may make a preparation of a freshly extracted living specimen that has not been anesthetized. Use method 2 in the Relaxation and Anesthetization section but immerse the snail in water, not alcohol. Pin the operculum to the wax bottom of the dissecting pan and open the mantle cavity with an oblique incision as described above, along the border between the rectum and gill. Mix carmine particles with condensed milk to make a thick pink suspension. You don’t need much. Place a drop of this suspension on the epitaenium and, at about 30X, watch ciliary currents move the milk and carmine particles transversely into the food groove where a mucous string of particles forms. The pink string with its entrained particles then moves anteriorly along the food groove into the cephalic food groove and then to the mouth. Let the preparation sit for 10-15 minutes then inspect the food groove where you will find a distinct cord of milky pink mucus extending into the cephalic food groove toward the mouth. Its movement is slow but discernable. <
In males the epitaenium sits atop a large rounded ridge, the prostate gland (= the seminal vesicle). The prostate extends obliquely from far posterior in the mantle cavity to a position at the base of the right tentacle (which is also the penis). In females there is no equivalent of the prostate gland and the epitaenium and food groove sits on the floor of the mantle cavity.
Excretory System
The excretory system of most prosobranchs consists of a single kidney (= nephridium) homologous to the left metanephridium of the pair present in t