Invertebrate Anatomy OnLine
Helix aspersa ©
Copyright 2001 by
This is one of many exercises available from Invertebrate Anatomy OnLine , an Internet laboratory manual for courses in Invertebrate Zoology. Additional exercises can be accessed by clicking on the links on the left. A glossary and chapters on supplies and laboratory techniques are also available. Terminology and phylogeny used in these exercises correspond to usage in the Invertebrate Zoology textbook by Ruppert, Fox, and Barnes (2004). Hyphenated figure callouts refer to figures in the textbook. Callouts that are not hyphenated refer to figures embedded in the exercise. The glossary includes terms from this textbook as well as the laboratory exercises.
Mollusca P, Gastropoda C, Euthyneura, Pulmonata sC, Stylommatophora O, Sigmurethra sO, Holopoda iO, Helicacea SF, Helicidae F (Fig 12-125)
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 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, 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, 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.
Most Recent molluscs are ganglioneurans, only the small taxa Aplacophora, Polyplacophora, and Monoplacophora are excluded. Neuron cell bodies are localized in ganglia.
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 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).
Euthyneurans are derived prosobranchs thought to share a common mesogastropod ancestor. Euthyneura includes the no longer recognized taxa Opisthobranchia and Pulmonata. The asymmetrical streptoneurous nervous system created by the torsion that defines Gastropoda is wholly or partly reversed by detorsion. The result is a secondarily symmetrical, euthyneurous nervous system. Euthyneurans are hermaphroditic, exhibit a tendency to reduce or loose the shell, tend to be bilaterally symmetrical, and usually lack an operculum.
The old subclass Pulmonata is no longer considered to be monophyletic and has been abandoned by zoologists as a formal taxon. Nevertheless it remains pedagogic convenience and is used here informally. Pulmonates are gastropods adapted for terrestrial and amphibious habitats. They are derived from prosobranch ancestors and have one atrium and one nephridium. The gill has been lost, however, and the mantle cavity is a heavily vascularized lung adapted for respiration in air. There is typically a coiled shell but some, the slugs, have reduced and internalized the shell and are not coiled and are secondarily symmetrical. Pulmonates are hermaphroditic and do not have an operculum. They are found in terrestrial habitats as well as near-shore, intertidal marine and freshwater environments.
Stylommatophora, the higher pulmonates, is the largest pulmonate taxon and includes the slugs and most terrestrial snails. (Aquatic pulmonates belong to Basommatophora.) Two pairs of tentacles are present and the eyes are at the tip of the posterior pair. Terrestrial snails are coiled, asymmetrical, and have a shell but slugs are uncoiled, symmetrical, and have lost or reduced the shell.
Helix, the edible terrestrial snail, is often used in invertebrate zoology courses as an example of pulmonate anatomy. Helix pomatia is the escargot and H. aspersa is the garden snail, a smaller species. The latter has been introduced to the west coast of North America where its abundance and voracious herbivorous appetite sometimes make vegetable gardening difficult. The exercise is based on H. aspersa but either species can be used. Color descriptions are for living or fresh specimens. Preserved animals are a uniform brown color.
Helix is a typical, shelled, coiled, torted pulmonate snail and as such is well chosen to serve as a representative pulmonate. The anatomy of Helix and the slugs is similar except thatHelix has a shell and is coiled. Because of these two differences, Helix is a more difficult dissection. The shell and coiling complicate the dissection and the body cavity is more crowded. If both are available, large slugs such as Limax, Arion, or Ariolimax are much easier to dissect. Further, slugs are much easier to anesthetize in a relaxed condition.
Helix is difficult to anesthetize in a relaxed and extended condition. The snail will retract into its shell when placed in almost any environment other than air or pure water. The older literature recommends drowning the animal by placing it in a jar of water over which there is no air space. The snail eventually succumbs in a more or less extended condition. They can also be killed and partly relaxed more quickly and humanely by placing them in 20% ethanol in water for about an hour. As is true of any snail, living or freshly killed specimens are much better than preserved. If living animals are fed a diet of dry pink cat food the gut will be conveniently color coded, the color of the food.
Place a preserved or relaxed, extended, freshly killed animal in a dish of water and examine its exterior.
Helix is coiled and torted. Its shell is thin, fragile, fat, and globular. It is brown with darker brown spiral stripes and is covered by a thin proteinaceous periostracum (Fig 1).
Figure 1. A garden snail, Helix aspersa, from Charleston, Oregon with the animal fully retracted. Pulmon11L.gif
The shell, like that of any coiled gastropod, is a hollow cone spiraled around a central axis, the columella. The columella cannot be seen, however, until you break the shell. Each complete turn of the cone is a whorl (Fig 1, 12-27A,B). Almost all of the shell of Helix is in the last whorl, which is known as the body whorl. Most of the body of the snail resides in the body whorl. There are a few much smaller whorls above the body whorl and these together make up the spire. The small spire sits atop the body whorl.
The body whorl opens to the exterior via a large opening, the aperture, from which the animal extends its head and foot. There is no operculum, the absence of which is a characteristic of pulmonates. The edge of the aperture is the aperture lip.
Examine the visible parts of the soft anatomy of your specimen. The body consists of a head, foot, and visceral mass. The head and foot can be extended from the aperture and should be visible now. The visceral mass always remains in the shell, cannot be extended, and is not visible now.
" Remove the animal from its shell by breaking bits of the shell away with a strong pair of forceps or a 4-inch C-clamp or small vise. Begin at the aperture and work your way up the body whorl. Be careful you do not damage any soft tissues. The animal is attached to the shell only by the columellar muscle. This strong white muscle has its origin on the columella well inside the aperture on the right side of the animal. It inserts on the foot. Find the columella, which is the central axis of the shell (Fig 12-27A). Use your scalpel or fine forceps to scrape (not cut) the origin of the muscle away from the columella. Gently pull on the foot to unwind the visceral mass out of the remains of the shell. Do not apply force or you will tear the visceral mass. If the snail does not unwind easily, reapply the C-clamp to an uncracked portion of the shell and continue cracking and removing pieces of the shell until you can completely detach the columellar muscle from the columella.
Clean away the pieces of the shell and immerse the snail in water again. Find
some familiar landmarks such as the head, foot, and visceral mass and orient yourself. The animal looks quite different without its shell and you may be temporarily disoriented. Spend a little time finding the major axes. Orientation of coiled gastropods is a little difficult due to their asymmetry. The animal is elongated along two axes, the anteroposterior and the dorsal ventral. Thehead and foot lie on the anteroposterior axis and are elongated along it. This axis is the axis of symmetry of the head and foot, which are bilaterally symmetrical. Neither head nor foot is affected by coiling or torsion.
The visceral mass, on the other hand, sits on the dorsal surface of the foot and is elongated along the dorsoventral axis. It coils up into the spiral shell and is asymmetrical.
Find anterior, posterior, dorsal, ventral, right, and left. Do this with the head and foot while ignoring the visceral mass. Remember that the head is anterior, the opposite end of the foot is posterior. The foot is ventral and the visceral mass is dorsal.
The head is at the anterior end of the animal and is well developed (Fig 2). It bears the mouth at its anterior end. The mouth is flanked by large fleshy lips. The head bears two pairs of retractile sensory tentacles at its anterior dorsal corners. The more dorsal and posterior cephalic tentacles are the larger and each bears an eye at its tip. The much smaller, more ventral and anterior oral tentacles have no eyes.
The common gonopore of the hermaphroditic reproductive system is located on the right side of the head. It is slightly posterior to the right cephalic tentacle (Fig 2). It is normally small and difficult to find but may be large and conspicuous if some of the reproductive apparatus, such as the penis, is protruding from it.
The foot is a long, wide, muscular organ with a smooth flat sole forming its ventral surface. The sole fits against the substratum and creeps along it. A mucus-secreting pedal gland opens between the head and the foot and secretes lubricating mucus onto the substratum in the path of the foot. This mucus creates the familiar slime trail of terrestrial snails and slugs.
Place the snail in the pan with the sole of the foot against the wax, as if it were crawling, and anchor the sole flat against the wax with two # 4 insect pins. Find the visceral mass if you have not already done so. It is the large, coiled mass of tissue sitting on the dorsal surface of the foot, posterior to the head. It was entirely hidden by the shell but is now visible. It is everything other than the head and foot.
Mantle and Mantle Cavity
The mantle is the body wall of the dorsal surface of the visceral mass. It encloses the mantle cavity and secretes the shell.
The mantle is folded to form a deep recess, the mantle cavity, immediately posterior to the head on the dorsal side of the animal. The mantle cavity of pulmonates is the lung, a large air space enclosed by the folded mantle. The edge anterior of the mantle of Helix forms a conspicuous mantle skirt, or collar, which you see as a high fleshy ridge encircling the dorsal and lateral body between the head and visceral mass (Fig 2). In life the skirt fits around the lip of the aperture of the shell and seals it. The fold of mantle is the roof of the lung (Fig 2). The unfolded dorsal body wall is its floor.
Figure 2. Helix aspersa viewed from the right side. The shell has been removed. Pulmon12La.gif
In most molluscs the gills are located in the mantle cavity but pulmonates have no gills. Instead the interior of the mantle cavity is vascularized to function as a lung for respiration in air. In the prosobranch gastropods there would be a large anterior opening into the mantle cavity between the mantle skirt and the body but in pulmonates this opening is reduced to a pneumostomeon the right side on or just below the edge of the skirt (Fig 2).
Although the opening to the lung is reduced, the cavity itself is very large and occupies most of the region at the base of the visceral mass. Its roof, which is the folded mantle, is very thin and translucent (in life).
The large anus lies beside the pneumostome on its postero-ventral border. Urinary wastes exit via a small nephridiopore in the ventral right corner of the pneumostome.
Many large organs are visible through the roof of the mantle cavity or just under the surface of the visceral mass of living specimens (Fig 2, 12-43). Unfortunately they are not apparent in preserved animals. If you have a living or fresh animal, you should find them now as they are useful landmarks for further explorations. Through the thin roof of the mantle cavity you can see the large, white, triangular nephridium at the posterior end of the mantle cavity (Fig 2). The translucent, white heart lies on the left side of the nephridium. The rectum, whose color depends on the diet, lies along the far right border of the lung (Fig 2). The white columellar muscle lies to the right of the rectum. The base of the body whorl is occupied by the large, brown left digestive cecum (Fig 2). A segment of the intestine can be seen imbedded in it. As you follow the coil of the visceral mass, the white albumen gland follows the left digestive cecum. The remainder of the coil is occupied by the right digestive cecum and the pale cream colored hermaphroditic gonad, the ovotestis.
" Use fine scissors to open the lung by making a longitudinal dorsal incision from the pneumostome to the posterior end of the cavity along the right side of the mantle. The cut will pass between the right side of the large bulging kidney and the tubular rectum. Do not cut into either of these organs. The incision will be a long one as the lung is very deep.
Figure 3. Dorsal view of Helix aspersa with the mantle cut and deflected posteriorly. Pulmon13La.gif
Make another cut transversely from the pneumostome to the left side of the mantle along the posterior edge of the skirt. Turn and cut posteriorly along the left side of the mantle to the posterior end of the lung. This cut will pass to the left of the heart and kidney. Do not cut anything other than the thin, translucent mantle. Avoid the organs in it and those below the cavity. Deflect the mantle, which is still attached to the body along its posterior edge. The lung is now open and accessible.
The space revealed by deflecting the mantle is the lung, or mantle cavity. Its roof is, of course, the folded mantle and its floor is the unfolded dorsal body wall, which is thin and transparent in this region (Fig 3, 12-43). Some internal organs can be seen through the floor. You will study them later. Note that there is no gill in the mantle cavity as there would be if this were a prosobranch.
Look at the inside surface of the roof of the lung. The large creamy white nephridium , or kidney, bulges from the mantle into the lung and you can now get a good look at it (Fig 3, 12-43).
The transparent pericardial cavity lies against the left side of the kidney, on the mantle (Fig 3, 12-43). It is enclosed in a membranous pericardium. In living or freshly killed specimens the heart is clearly visible within the pericardial cavity but in preserved material the heart will not be visible until the pericardium is opened later in the exercise. The heart consists of an anterioratrium and a slightly larger, opaque, posterior ventricle.
A conspicuous intricate pulmonary plexus of blood vessels on the lung roof drains into the anterior end of the atrium by a large efferent pulmonary vessel (Fig 3, 12-43). A large aortaexits the posterior end of the ventricle (Fig 3). It immediately penetrates the dorsal body wall and you will not see it now.
The tubular rectum runs along the right edge of the mantle cavity at the line of junction between the roof and floor of the lung (Fig 3, 12-43). It begins at the posterior end of the lung, where it emerges from the visceral mass, and runs anteriorly to end at the anus lying on the lower right edge of the pneumostome.
The ureter leaves the nephridium and runs along the outside (right) edge of the rectum (Fig 12-43). It empties at the pneumostome via the nephridiopore.
" Open the body cavity (which is a hemocoel) with a longitudinal, middorsal incision, through the body wall. Make this cut with fine scissors, from the mouth posteriorly, through the mantle skirt, and along the floor of the lung. Be careful that you cut no deeper than the body wall. The body wall of the head is thick and tough but that of the floor of the lung is very thin. The skirt is also thick. Do not damage the internal organs, most of which belong to the digestive and reproductive systems. Pin the body wall aside. The cavity you have revealed is the hemocoeland it is tightly packed with organs. In life they would be immersed in blood.
Find the following structures to use as landmarks. In the head find the buccal mass. This large, ovoid mass of muscle contains the pharynx and radula. You will open it later. The large, dark (its color depends on that of the last meal), tubular crop lies on the left and extends the length of the hemocoel. The complex, mostly white reproductive system occupies all the space on the right.
" Relocate the pericardial cavity in the roof of the mantle and use fine scissors to open it to reveal the atrium and ventricle more clearly. The atrium is anterior and the ventricle is posterior. The efferent pulmonary vessel drains the pulmonary plexus into the anterior end of the atrium. Find the aorta exiting the ventricle posteriorly and trace it through the body wall into the hemocoel. It leads to an extensive system of vessels that extend throughout the hemocoel to supply its organs. Trace some of the vessels to their targets if you wish. Any further study of this system should be done now, before the vessels are destroyed.
Free the reproductive system from its membranous connections with the dorsal body wall and crop and move it to the right, out of the way.
The narrow, tubular esophagus exits the buccal mass and turns to the left to connect with the much wider crop. The brain is a large circumesophageal nerve ring around the esophagus just posterior to the posterior buccal mass (Fig 4, 12-54B, 12-43).
The nerve ring is enclosed in a sheath of connective tissue. The sheath is a very large, easily located, flat band. The ganglia and connectives of the nerve ring are inside the sheath and, in preserved specimens, cannot be seen without further dissection.
The nerve ring includes a pair of large dorsal cerebral ganglia which are connected to each other by a short cerebral commissure passing across the midline. A large connective exits each cerebral ganglion and passes ventrally and posteriorly to a cluster of ganglia ventral to the esophagus. Included in this cluster are the pleural, pedal, esophageal (= parietal), and even the visceral ganglia (Fig 12-43). Many nerves exit the ring. The large cephalic aorta can be seen penetrating the ventral cluster of ganglia on its way to the head. The optic nerve from the cephalic tentacles can be seen entering the ring.
The columellar muscle originates on the columella of the shell and extends from there into the body where it divides into several long muscles which run along the floor of the hemocoel. Most of these are long, white, flat, and straplike and lie under the reproductive organs and crop.
Find the buccal retractor muscles. These two branches of the columellar muscle pass from their origin at the columella through the nerve ring to insert on the buccal mass. Their action is to withdraw the buccal mass and head into the shell and body.
Two other slips of the columellar muscle, the cephalic tentacle retractor muscles, run to the two cephalic tentacles. Their action is to withdraw the tentacles into the head. Near the tentacles they are dark grey or black. They do not pass through the nerve ring.
Yet another pair of retractor muscles runs to the oral tentacles and there is also a penis retractor muscle. The remainder of the columellar muscle fans out into numerous paired slips to all regions of the foot. These are the pedal retractor muscles and their action is to withdraw the foot into the shell when the snail is threatened.
" Relocate the buccal mass in the center of the head (Fig 4). Use fine scissors to open it with a longitudinal, middorsal incision beginning at the mouth. Be careful that you do not cut the nerve ring which encircles its posterior end. This incision will reveal the lumen of the anterior gut.
The mouth opens into a small anterior buccal cavity equipped with a single, dark, heavy, toothed jaw. The jaw is located dorsally over the opening of the mouth into the buccal cavity. You cut through it when you opened the buccal mass.
Figure 4. Dorsal dissection of Helix aspersa with the mantle cavity and hemocoel open. The mantle roof has been moved to the left and reproductive system to the right for clarity. pulmon14La.gif
Most of the interior of the buccal cavity is the pharynx. The radula lies on the floor of the pharynx. The radula is a wide, golden brown, thin, transparent sheet of tough tissue which bears rows of numerous tiny chitinous teeth. The radula is situated on the dorsal surface of the connective tissue odontophore, which contributes to the floor of the pharynx. The many muscles of the buccal mass operate the radula and odontophore and include retractor and protractor muscles for both.
>1a. Remove the radula and make a wetmount of it for study with the compound microscope. <
A short, narrow esophagus exits the posterior end of the pharynx, passes through the nerve ring, turns to the left and widens to become the anterior end of the crop (Fig 4, 12-43). The crop is a large, thin-walled storage organ extending the entire length of the hemocoel. The snail's last meal may be visible inside it.
A pair of diffuse, white (in life) salivary glands partially cover the walls of the anterior end of the crop (Fig 4, 12-43). In preserved specimens the salivary glands are about the same beige color as the crop. Each gland drains to the pharynx via a long salivary duct that passes through the nerve ring.
Follow the crop posteriorly, past the origin of the columellar muscle. In this region it is obscured by the large, white albumen gland of the reproductive system and the two dark browndigestive ceca. The right digestive cecum is in the upper, small whorls of the visceral mass and, in fact, accounts for most of the volume of the upper whorls. The pale cream ovotestis is also in the upper whorls located along the inside curve of the spiral. The albumen gland is also present in the spiral of the visceral mass but is lower down, in the middle region and base of the visceral mass. The left digestive cecum lies at the base of the visceral mass and extends into the mantle roof to lie against the kidney.
" Use fine scissors to cut connective tissue as necessary to separate the albumen gland from the left digestive cecum. Cut posteriorly along the left side of the rectum, following the rectum until it disappears from view into the visceral mass. This will expose most of the albumen gland. Be sure you do not cut into the gland.
" Cut the membranous connective tissue holding the albumen gland to the crop but do not cut any organs. When all these connections are severed, grasp the albumen gland gently with forceps and pull it out of its pocket in the middle whorls of the visceral mass. Do not break or damage the convoluted, white hermaphroditic duct which runs from the ovotestis to the albumen gland (Fig 4, 12-43). You cannot see all of the ovotestis yet. The hermaphroditic duct is lost from view under some of the branches of the columellar muscle.
Between the two digestive ceca the crop expands to become the stomach, into which the digestive ceca open (Fig 4, 12-43). The stomach narrows to become the intestine which is also embedded in the left digestive cecum. It makes a large loop as it passes through the gland. The intestine emerges from the digestive cecum as the rectum, which you have already seen. The rectum runs anteriorly along the right side of the mantle cavity to open at the anus.
Pulmonates are hermaphroditic with internal fertilization. Both conditions require modifications of the reproductive system and that of Helix is one of the most complicated you will encounter. Fortunately almost all parts of it are in the hemocoel and are relatively easily found and identified.
The protandric (male first then female) ovotestis is located at the top of the visceral mass high in the spire (Fig 2, 12-43). It is embedded in the digestive cecum and is pale creamy-white (in life) or dark brown (preserved). In preserved specimens the ovotestis is darker than the digestive cecum whereas the opposite is true of fresh material. The degree of pigmentation of pulmonate gonads depends on the reproductive condition of the individual. A small portion of the gonad can be seen (barely) without further dissection on the surface of the visceral mass in the groove between the smallest whorls. It is almost completely surrounded by the right digestive cecum. You will have to dig into the right digestive cecum to see more of the ovotestis but you should not do so at this time.
Relocate the albumen gland and separate it from its surrounding tissues if you have not already done so. This gland secretes a nutritive material around the fertilized egg. Its size varies with changing reproductive condition but it can be quite large. A small, but conspicuous (in life), white, convoluted hermaphroditic duct extends from the ovotestis to the albumen gland (Fig 4). Autosperm are stored in the hermaphroditic duct which thus functions as a seminal vesicle.
Fertilization occurs in a fertilization chamber embedded in the albumen gland (at the point where the hermaphroditic duct, albumen gland duct and sperm-oviduct join.)
Anteriorly the albumen gland is continuous with a very thick and conspicuous spermoviduct or common gonoduct (Fig 4, 12-43). This is a combined sperm duct and oviduct running side by side. The large sacculated side of the duct is the oviduct whereas the smaller bright white inner margin is the sperm duct. The lumina of the two ducts are not completely separated from each other. The oviduct secretes a calcareous shell around the fertilized egg as it passes downstream.
The reproductive plumbing is complicated by the presence of two other ducts partially associated with the exterior of the spermoviduct. These are both associated with the copulatory bursa (= spermatheca), a small spherical sac attached to the wall of the upper end of the spermoviduct. The bursa stores allosperm until needed for fertilization.
The bursal duct is much easier to find than the bursa itself (Fig 4, 12-43). It exits the bursa and runs anteriorly along the wall of the spermoviduct but is not functionally associated with it. Similarly, a blind tubular diverticulum, the bursal cecum, arises on the other side of the spermoviduct and follows a twisting course anteriorly eventually to join the bursal duct. The sperm duct is less obvious than either of these and appears as a white band in the wall of the spermoviduct, not as an independent duct.
As the spermoviduct nears the head it divides into a separate oviduct and sperm duct. The oviduct joins the distal end of the spermathecal duct which then joins the large, pyriform, muscular dart sac to form the short vagina (Fig 4, 12-43). The dart sac secretes and stores a calcareous dart which is stabbed into the snail's partner, presumably to stimulate copulation. A pair of large, branched mucous glands arises from the dart sac. The mucus glands secrete mucus for lubrication.
The sperm duct exits the spermoviduct as a discrete independent duct and turns medially, passes over the base of the dart sac and vagina and under the right cephalic tentacle retractor muscle, then expands in diameter to become the penis (Fig 4, 12-43). During copulation, the penis is inserted into the gonopore and vagina and delivers sperm to the partner.
At the point where the penis begins to expand it is joined by a long, slender, blind diverticulum, the flagellum (Fig 4, 12-43). Sperm from the sperm duct are packaged into spermatophores in the flagellum. The penis joins the vagina to form a short common chamber which opens to the exterior through the common gonopore. You saw the gonopore earlier when you studied the right side of the head.
>1b. Observe the behavior of a living animal in a terrarium. Note how the animal crawls on the sole of its foot. Waves of muscular contraction move from posterior to anterior along the foot to move the animal. Such waves are said to be prograde since they move in the same direction as the animal. Retrograde waves move in the opposite direction.
Watch the sole of the foot of a snail through the glass wall of a terrarium wall as the snail crawls across the walls of a terrarium. Can you tell if the waves are retrograde or prograde?
Bullough WS . 1958. Practical Invertebrate Anatomy, 2 nd ed. MacMillan, London. 483p.
Hyman LH . 1967. The Invertebrates, Mollusca, vol. VI. McGraw-Hill, New York. 792p.
Fretter V, Peake J. 1975. Pulmonates 1. Functional Anatomy and Physiology. Academic Press, New York. 417p.
Rowett HGQ . 1957. Dissection Guides V. Invertebrates. Reinhart, New York. 56p.
Ruppert EE, Fox RS, Barnes RB. 2004. Invertebrate Zoology, A functional evolutionary approach, 7 th ed. Brooks Cole Thomson, Belmont CA. 963 pp.
Slides and coverglasses
Living or preserved Helix
20% ethanol in pondwater
Small dissecting pan
#4 stainless steel insect pins