By Avril Bourquin
Some Science Editing by Ross Mayhew
May, 2000

Early Beginnings:

   The time is now about 600 million years ago and the first molluscs have made their appearance on our world. About 100 million years later, during the Ordovician period, at least six of the seven classes of molluscs represented today were present. Many of these first molluscs were but simple, worm-like animals, having segments similar to what we find in annelid worms and arthropods. These first molluscs crawled about the primeval seas, probing for and eating microscopic bits of food.

     The great landmass of Pangaea slowly deposits dissolved salts and other chemicals into the ocean. The first primitive molluscs in these oceans now digest these chemicals and begin to use the nutrients to build themselves protective shelters (shells) against their hostile environment. As Pangaea breaks apart around 200 million years ago, the world's great continents slowly migrate, due to plate tectonics, and we begin to recognize the world continents as they are today.

     Over time, the molluscs flourish and evolve to fit newly developing habitats. Fossil records show some groups ("taxa") growing larger, some smaller. Some grow spiny, others, shiny. Some coil tighter, some looser. Some coil left to right while others loose their coil all together. Some even loose their shells completely. Some, like the ammonids, evolve into huge numbers of species, and then mysteriously disappear forever only to be found as fossils. About 400 Million years ago, some of these molluscs, first the bivalves, begin to inhabit the worlds freshwater streams and lakes. It took at least another 300 million years for certain gastropods to evolve to where they were capable of populating all land and freshwater water habitats.

     During the last million years, the land and freshwater molluscs have evolved very rapidly; however, some groups of marine molluscs appear to be decreasing in the number of species existing.

     Today, molluscs live in almost all parts of the world. From the deepest ocean trenches to high up on our mountains, molluscs have found their niche. The number of living species ranges from a very conservative 50,000 according to Brusca & Brusca (Invertebrates 1990) to 60,00 in Rupert & Barnes Invertebrate Zoology (sixth edition 1994) to 100,000 in Kozloff's Invertebrates (1990). That said, it is quite likely that up to half a million species will eventually be formally discovered (see the article on How to name a Species, for how this is done!), since many environments and the deeper parts of the sea-floor are very poorly known even today!  Classification and taxonomy of molluscs can vary widely depending on what school of thought.

     One thing does remain constant in all molluscs however; - to survive all molluscs must have moisture. To stay alive, they must keep their soft bodies moist at all times and for some like those which live in hot dry deserts environments, this is done by curling up in their shell, secreting a mucous plug and staying holed up until the next bit of moisture comes along.

What is a Mollusc?

  The word "mollusc" or "mollusk" (both are correct) is derived from the Latin word mollis meaning "soft". The study of molluscs, "malacology", comes from the Greek word for soft, malacos. The term "conchology" is also used for the study of molluscs; however, it is usually applied to those that study the shell only.

     Molluscs, in general, are soft-bodied animals that usually produce an external skeleton (called an "exoskeleton") we call a Shell, which is composed of a limey material: calcium carbonate (CaCO3) The shell serves both protective, and supportive purposes. The one feature common to all molluscs is the presence of a fleshy mantle. This is a fold or lobe (or a pair of them) of fleshy material, which secretes, modifies and lines the shell. Members of all classes except the bivalves possess a ribbon-like set of hooked teeth called radula. These they rasp (think of a fingernail file) back and forth over their food much the same idea as a cat lapping up milk: Vegetarian species use them to scrape algae off rocks and other substrates, while most molluscan carnivores use them to penetrate the surface of their prey - even when that is a decent thickness of shell! In the superfamily Conoidea, which includes the Cones and the largest family in the mollusc world, the Turridae, the radula is specialized into a form of miniature "harpoon", which is used to spear prey, and in many cases, to deliver powerful neurotoxins, to paralyze their hapless victims. Most molluscs have a well-defined nervous system with a primitive brain. Molluscs have a circulatory system and most have a two-chambered heart.  Their digestive system usually includes a jaw, pharynx, esophagus, stomach, intestine and anus. They have a reproductive system that produces eggs and/or sperm.  Most gastropods and cephalopods have eyes and tentacles.

Now Let's Take a More Detailed Look at The Phylum Mollusca

A phylum is usually defined as group of animals having several features common to all or most of    its members.  The following features are common amongst most molluscs:

• A mantle that secretes calcium carbonate in the form of spicules or shell
• A mantle cavity where respiration occurs, usually through the ctenidium (gill) in aquatic forms, or through the mantle wall in terrestrial molluscs.  This is also where excretory and reproductive organs discharge as well.
• A body divided into three regions, the head, foot, and visceral mass
• A radula, which is a ribbon of teeth used in feeding
• Three coelomic (body cavity) spaces, for kidney, heart and gonad
• A Foot used in locomotion and often for digging into the sand or mud bottom where many live.

     Biologists use various methods for estimating how closely species are related to each other.  They look at comparative anatomy, genetics and paleontology (the study of fossil organisms) to help form their theories. Changes within a population (a group of organisms of one species) generally occur due to divergence and speciation.

     Divergence within a phylum can occur whenever the population is split into two or more groups with no chance of interbreeding.  Divergence is generally brought about by such events as habitat changes or competition for food.

     Speciation can result from reproductive isolation (populations can be physically isolated, as in many marine species, without being isolated reproductively, due to larval (veliger) stages which can drift for long distances, thus effectively "connecting" geographically remote populations. On the other hand, behavioral, morphological or reproductive differences in a small segment of a population can gradually lead to reproductive isolation, without much physical segregation. These reproductively separated populations will adapt to different conditions in different regions, via "natural selection" (i.e., survival of the fittest!).  They may develop different mating behaviors or breeding seasons, or they may accumulate enough genetic differences to render egg and sperm incompatible.  It is due to an accumulation of these changes and other morphological and genetic differences that we have the seven classes of molluscs today. This seemingly advanced degree of differentiation, however, took place hundreds of millions of years ago: by the middle of the Ordovician period, all the shell-bearing classes (6) of mollusc are represented in the fossil record.

     Now, let's take an even closer look at these seven classes of molluscs. We'll cover the basic anatomy and physiology and behavior of each group, and a variety of other interesting facts about each. Enjoy!

   The two first two classes we will discuss - the Aplacophoran and the Polyplacophoran, are often regarded by some as Subclasses of a larger Class called Amphineura
(Amphineura: (am-phi-neur-a) Latin:  amphi =both.   neura =nerve).
However, we will follow the crowd (i.e., the majority of scientists), and treat them separately.

 (poly-plac-o-phor-a  (lor-i-cat-a))
Latin meaning:  poly = many    plac = plate    phor = carry, i.e.: bearer of many plates.

NOTES:  1) There are a lot of rather "technical", or scientific terms in theses class-descriptions.  Fear not - most of them will be hooked up to a glossary by the time things are finished, so the reader will just have to click on them to see a definition and derivation.  Meanwhile, we suggest a good dictionary J)
2)  Images (diagrams and photographs) will also be added to this part of the site, as well as some links to relevant sites. 

     The polyplacophorans, commonly known as chitons, are often considered by scientists to be the most primitive of all existing molluscs.  Strictly marine, the majority of the chiton species inhabit rocky seashore environments where their low dome-shaped shells are well suited to withstanding the violent serge of ocean waves. They all cling tenaciously to the hard substratum and if dislodged from its rock, will roll up into a ball to protect their fleshy under surface.  This also allows it to roll around safely in the waves until it can reattach itself to a rock. Most chitons are herbivorous; however a few are predatory. They are nocturnal in behavior. 

Many cultures use various species of chitons as food and as fish bait (NOTE: For more of man's uses of Molluscs, see the Man and Mollusc article!)

     Chitons are generally bilaterally symmetrical with an ovoid, flattened body. The most distinctive characteristic of chitons is their eight-piece shell.  Each of these eight plates is quite similar, except for the first and last (the cephalic and anal plates).  The posterior margin of each plate projects backwards, and the anterior lateral margins of each one bears a large wing that projects forward.  These projections then fit beneath the plate immediately in front, and each plate overlaps the plate behind: a very tight arrangement - perfect for defense!  Except for the posterior edge, a reflexed fold of mantle tissue covers the margins of each plate.  In some chitons (such as the genus Amicula), the mantle totally covers the plates. The girdle is very heavy and extends beyond the lateral margins of the plates.  The girdle surface may be naked and smooth or covered by scales, hairs  or calcareous spines.  These so-called hairs can be so long and dense that the animal takes on a mossy or shaggy appearance. Because of the transverse lie and articulation of these plates a chiton can live on a sharply curved surface.  If a chiton becomes dislodged (this has not been observed to ever have happened on purpose) from its hard surfaced home, it can roll up into a ball.  This could be a defensive mechanism to prevent damaging its softer body parts as it rolls in the surge of ocean waves until it can successfully relocate onto another suitable surface.

     Chitons have a broad flat foot, which occupies most of the ventral surface of the animal.  It serves both for locomotion and adhesion.  Being very sedentary by nature, chitons, especially the older individuals, will stay in a very small area all their lives if an adequate food supply is available. For some species, this could be an area of as small as six square feet. 

     The foot secretes a small amount of mucous and propulsion is accomplished entirely by muscular contraction. 

     Both the foot and the girdle affect adhesion.  Ordinarily, adhesion is accomplished just by means of the foot; however, when disturbed, the girdle clamps down on the hard substratum and the inner margin is raised.  This creates a vacuum that enables the chiton to grip the surface with great tenacity.  This is also the reason that chitons prefer smooth hard surfaces on which to live (rocks, shells of other molluscs, lobster traps and other sunken wood, anchors or other metal, etc.  Interestingly enough, glass does not make a good substrate, because it is TOO smooth, which makes it difficult to get a truly secure grip.)

     Because chitons have ventrally flattened bodies and due to the fact that they adhere to hard substrates, their mantle cavity has had to extend forward as a groove on both sides of their body.  This groove runs between their foot and the mantle edge (trust me, this is all a lot easier to picture with a picture, which we'll supply in good time!!). The margins of the Chiton's mantle are held down tightly to the hard substratum making these grooves into a closed chamber.  A large number of small, paired gills are arranged within these two mantle grooves.  The number of pairs of gills varies from species to species and can even vary within a species itself.  These gills hang down from the top, or roof, of the pallial grooves and their tips touch the lower margin of their foot, which divide the grove into a ventro-lateral inhalent chamber at the front of the chiton and a dorsal-medial exhalent chamber at the end of the chiton.  As the anterior mantle margins are raised, two inhalent openings are formed through which water flows.  This water then flows along the length of the groves through the gills and into the two dorsal exhalent chambers.  The two exhalent water currents then converge posteriorly and pass to the outside through two exhalent openings that are created by the locally raised mantle.  

     The pericardial cavity, which contains the two-chambered heart, is large and is located beneath the last two shell plates.  A single pair of auricles collects all the blood from the gills and pumps it to the lone ventricle.

     The Chiton's nervous system is very primitive.  There is no brain, only a poorly developed ganglia and often that is absent as well (so, while it is certainly unethical to torture a chiton, they aren't capable of feeling "pain" as we know it, and most certainly have no thoughts or emotions of any kind!).  Neurons are simply scattered along the length of nerve cords.  A nerve ring surrounds the buccal cavity and subradular organ.  A posterior nerve ring gives rise to a pair of pedal nerve chords, which innervate the foot muscle.   A large pair of pallial visceral nerve chords completes the nerve ring. Some families do have specialized eyes called the aesthetes.  These are mantle sensory cells that penetrate the articulamentum of the shell and are lodged within vertical canals leading to the outside of the shell.  In some chitons these are only tactile sensory organs.  In others they are more specialized - actually taking on the properties of an eye, complete with a cornea, lens and retina!  These chitons may possess thousands of these little "eyes" which appear as minute black spots on their shells.

     Most chitons are herbivorous - feeding on unicellular and multicellular algae. They scrape this alga off of the rocks and other substrates on which they live, by means of their hard, raspy radular ribbon. The mouth is located at the anterior end, in front of the foot and opens into a buccal cavity containing the radula  - and a smaller, more ventral subradular sac that in turn contains a sensory structure.  A pair of salivary glands secrete through an opening in the wall of the buccal (i.e., mouth) cavity.  When the chiton feeds, the subradular organ is first protruded and held against the surface.  If food is sensed, the odontophore with its radula project from the mouth and begin to scrape the algae off the substrate.  When the radula is retracted, these food particles are pressed against the roof of the buccal cavity and carried into the esophagus.  Saliva is added to this food at this point: it contains no digestive enzymes and acts merely as a lubricant for transporting the food particles.  This mucus and food slurry is then carried along the ciliated esophagus towards the stomach. Along this passage, this food is mixed with amylase (a digestive enzyme), secreted by two large esophageal glands that enter the esophagus through two ducts.  The esophagus then enters an irregularly shaped stomach that contains a large ventral sac.  Further digestive enzymes are added to this slurry and aided with the contractions of the stomach; the food is then churned and dissolved into absorbable nutrients for the chiton.  These nutrients are then passed along to the intestine where food absorption takes place.  All waste materials pass through a sphincter into the posterior intestine where liquids are further absorbed and solid waste matter is it is compacted and passed along its mucous lined interior.  It is then divided into pellets; further compaction takes place until it is finally passed through the anus (which opens at the midline just behind the posterior margin of the foot), as small, solid fecal pellets.

     The two nephridia (a kind of primitive kidney, responsible for renal function) are quite large, and extend anteriorly on each side of the body as long U-shaped tubes. They are responsible for removing waste from the blood.  This liquid waste is passed out through two nephridiopores into the pallial grove located on each side between the more posterior pairs of gills.

     All chitons are dioecious (i.e., they have two sexes).  Both males and females possess a single median organ that is located in front of the pericardial cavity under the middle shell plates.  Two gonoducts open directly to the outside.  A gonopore is located in each pallial groove, in front of the nephridiopore.  There is no actual copulation, and fertilization, in most cases, occurs in the mantle cavity:  Sperm leaves the male in its exhaled water and enter the female via inhaled water. The fertilized eggs are usually just shed into the surrounding water; however, in some species the eggs are retained inside the female s mantle cavity and she gives birth to the live young that have developed within her oviducts. Since Chitons are gregarious by nature, this form of reproduction is quite successful - and no romance is required.

 (mono-plac-o-phora)

Latin:  mono=one    plac=plate    phor=carry : bearing a single single plate (shell)

     Until the mid nineteen hundreds, this class, often call the "gastroverms", were thought to be extinct and appeared only in Cambrian fossil records.  Then in 1952 scientists discovered ten living specimens while on the Danish Galathea  expedition.  Two years later on the same expedition, two more species were discovered.  Now we recognize about a dozen living species.

    These strange, limpet-shaped molluscs are segmented like worms. In each segment of the creature the internal vital organs are duplicated. They are indeed primitive in biology and as such tend to live only in the deeper ocean areas where they are away from the more advanced and active predators.

     Monoplacophorans possess a single, large, bilateral shell.  The shell is a simple depressed limpet or disk -shaped valve, less than 25 millimeters across usually and is often thin and fragile. The earliest developed part is a coiled chamber. The outer surface of the adult is covered with a protective horny periostracum, or sheath. On the inner surface of the shell, there are significant paired muscle scars, suggesting segmentation.

     Monoplacophorans possess a foot, round in outline and not very muscular, which is responsible for locomotion.   The muscular action is similar to that of the polyplacophorans.  (Please refer to the Polyplacophora for info on this.).

     Running along the mantle gutter cavity on either side of the body are five or six pairs of gills, however, filaments only exist on one side of the gill axis.

     Monoplacophorans possess a single ventricle and two auricles for circulating the blood per body segment.  The first pair of auricles receives the blood from the first four pair of gills.  The pericardium is paired and the heart lies between the two divisions.

     The head of Monoplacophorans is much reduced in size lacks true eyes and tentacles.

     The nervous system of this class is very similar to that of the Polyplacophorans.  (Please refer to
the data on Polyplacophora for that information!).

     The digestive system is very similar to that of the gastropods.  (Please refer to that data for details). The difference is that behind the mouth is a curious cluster of frond-like appendages that serve to push the food into the pharynx.  Also the mouth is located in front of the foot and the anus is located posteriorly in the pallial groove.  From the nature of their radula ribbon of teeth, it can be postulated that they ingest mud or bottom detritus. The radula is actually upside down as compared to the radula of other
molluscs.

     Six pair of nephridia are present and are arranged in series (metamerically) on each side of the body.  The nephridiopores of the last five pair open near the five-gill pairs.

     The sexes are separate, and two pair of gonads are located in the middle of the body.  Each gonad is connected by a duct to one of the two pairs of nephridia (kidneys), which are located in the middle of the body.

     There is SO MUCH still to be learned about these inhabitants of the deep, and if you become a marine biologist, some day you could be the one to shed light into some of the darker corners or our knowledge of these and other little-studied taxa!

Latin:  (A-plac-o-phor-a, from the Latin a=without   plac=plate   phor=carry: i.e., not bearing a shell)

Chaetoderma	Neomenia

 The Aplacophora are a small group of molluscs, which have deviated from the normal molluscan form.  There are approximately 100 known species living today.  They are rather worm-like and average about an inch (2.5 cms.) in length.   Most live in deep water, except a few more northern species.  One group bury themselves into the sand and mud of the ocean bottom where they feed on annelids and other small invertebrates.  The rest of the known aplacophorans parasitize hydroids and other corals.

The shell is absent and there is no fossil record to suggest that any members of the class ever had one. 

The fleshy mantle does not produce a shell in aplacophorans but is embedded with calcareous spicules, presumably to make them less palatable to predators.  Their body shape is slightly oval and flat in appearance; however, in one group of aplacophorans, the order Chaetodermomorpha, the mantle has fused to form a cylindrical body (note: we have Chaetoderms in Nova Scotia - in St. Margaret's Bay (-Ross)!) 

Aplacophorans only possess a trace of a mantle cavity.  Gills are lacking in many of the species while others possess only secondary gills.  The cloaca, a cavity into which the anus and a pair of nephridiopores empty, is possibly a remnant of the mantle cavity.

The foot is either virtually absent or vestigial:  a simple ventral fold:  It is much reduced and has become just a tiny median ventral ridge lying in a small longitudinal groove.  This means that the Aplacophora have no viable means of locomotion.

In the aplacophorans this consists of a simple cerebral ganglion and a lateral nerve cord. There are no specialized sense organs such as eyespots or electrical or chemical sensors.

The head is poorly defined in all aplacophora, and their visceral mass consists of a very simple and straight digestive system.  Food taken in passes through the circumpharyngeal muscle into the oral cavity where a radula rasps it (the radula is usually present although it is often somewhat modified).  The fine food particles then pass into a single midgut organ that consists of a stomach and digestive gland.  A short intestine absorbs nutrients before the waste passes into the cloaca.

Aplacophorans are either hermaphroditic (i.e., self-fertilizing, as in the family Proneomenia) or dioecious (i.e., having two separate sexes, as in the family Chaetoderma) (Two quite different sets of "family values"J).  Copulation never occurs: males release sperm freely into the water.  The females also release their eggs into the water, or hold them within their mantle cavity.  In the latter case, inhalent water drawn into the female's mantle cavity contains sperm, which fertilizes the eggs held there.

Latin:  (sca-phoda: from the Latin  scaphe=boat , and pous=foot:  boat - foot!)

   All scaphoda species, a mere 200 or so, are marine inhabitants that live partially buried in sand or gravel all their lives.  The tusk or tooth shells, as they are more commonly known, have the simplest shell structure and anatomy of all the molluscs. They show very little variation in structure except that the members of the Dentalium resemble elephant tusks, while those of the Cadulus class look like swollen cucumbers open at each end. The number and shape of the longitudinal ribs along with the coloration and curious slots appearing along the edge of the smaller posterior end of their shell all help to distinguish the various species of tusk shells.

They are relatively inactive creatures with a low metabolic rate and a very simple anatomy. Scaphopods generally burrow in the sand at depths ranging from 18(6 meters) to 600 feet (200 meters), however, a few do inhabit shallower waters and some have even been found in the oceans' deepest trenches.  

     The western tribes of North American Indians once used tusk shells extensively, first as necklaces and later as money belts (Wampum) to be traded with other tribes, and the white man. 

     The shells of most species average between one to two inches (2.5 cams. ­ 5 cms.) in length; ranging from 4 mm (Cadulus mayori) to a maximum of about 150mm (6") (Dentalium vernedei Sowerby, 1860).  The name "tusk shell" is derived from the fact that it resembles the shape of an elephant's tusk.  The shell is usually an elongated, cylindrical tube, open at both ends, and slightly curved. However, some of the scaphoda shells are shaped more like a swollen cucumber.  The shell is usually heavily ribbed and has small slits at the narrowest end. Colours range from white & off-white, cream, brown to an attractive variety of greens. 

     The foot extends from the larger end of the shell and is spade or cone shaped.  It is projected downwards, so the animal can burrow with it.  It also serves as a means of an anchor for the animal.  Finally, by the contracting and expanding motions, the foot also keeps water passing in and out of the posterior half of the mantle cavity which in turn causes blood circulation.

     The mantle cavity is large and extends the entire length of the ventral surface of the scaphopod.  Water slowly enters this cavity as a result of ciliate action (many small cilia all beating in the same direction).  The cilia are concentrated on the vertical ridges of the mantle wall, approximately midway on the animal.  The remaining mantle wall is only weakly ciliated.  There are no gills for respiration.  Gases are diffused directly through the mantle wall.  After about 10 to 12 minutes, a violent muscular contraction expels the water from the same inhalent opening. 

     The circulatory system is reduced to a simple system of blood sinuses (cavities).  There is no heart: the foot keeps the blood circulating, as described above, and gases are simply diffused through the cell walls lining these sinuses. 

     The usual molluscan sense organs such as eyes, tentacle and nephridia are absent in Scaphopods.  They do however have the familiar cerebral, pleural, pedal, and visceral ganglia and their corresponding nerve chords common to most molluscs. 

     The feeding mechanism of the Scaphopods is complicated and unique. Their food often consists of a microscopic family of one-celled test-forming organisms called foraminifer (a good for a site to visit), some of which live on sand or silt, which they glued together to form their tests (which function as shells).  The Scaphopod head is reduced to a short conical projection or proboscus, bearing the mouth.  They bury themselves head down in the sand or mud - like politicians avoiding an issue!. 

     On each side of the head are lobes bearing a large number of thread-like appendages called captacula.  Each of these tentacle-like food-gatherers has an adhesive (i.e., sticky) knob at its end. The captacula are extended into the surrounding sand or mud to capture food and bring it to the mouth.  The buccal cavity contains a well-developed radula with large flattened teeth.  The radula aids in ingesting, and shreds their food. 

     The stomach and digestive gland are located in the middle of the body.  The intestine then extends anteriorly, and then loops around to open through the anus into the mantle cavity.  Details of scaphoda digestion and absorption are still unknown: they are a complicated lot!! (Want a Masters thesis someday???) 

     Scaphopods possess a pair of nephridia (renal organs), which drain out of the body through nephridiopores located near the anus. 

     Scaphopods are dioecious (Two sexes, i.e.).  Eggs and sperm reach the outside of their body through the nephridiopores. The eggs are shed singly and are planktonic (i.e., free-swimming).  Fertilization takes place when the sperm and egg meet by chance in the water surrounding a spawning couple or group.  The larvae drift freely in the water-column until they eventually settle down on the ocean's bottom. 

Latin: gas-tro-pod-a:  Latin meaning:  gaster=stomach  pous=foot :  stomach - foot!)

         Cypraecassis rufa
 (by permission of Guido Poppe)
     The gastropoda is the largest and most certainly the best-known Class of all the molluscs.  They are the most successful of the molluscan classes, and occupy almost every habitat on earth, from desserts to high mountains, fields, forests, lakes, streams and oceans - and most probably your back yard!!  It is the only class to contain species that have ventured permanently on to land.  (To do this, snails evolved an efficient gliding foot, eyes, an aggressive eating mechanism and a pulmonary system for breathing.)  Gastropods also inhabit every niche in the ocean from the intertidal zone to the deepest ocean trenches. Over 15,000 fossil forms have been described and over 40,000 species exist today.  They are, scientists theorize, now at the peak of their evolutionary development. 

     Gastropods exhibit the least change from the ancestral molluscan plan of all the molluscs. The pretorsion (pre = before, torsion = twisting) shell of the ancestral spiraled gastropods resembled a coiled garden hose flat lying on the ground.  This plano-spiraled (i.e., coiled all in one plane - flat!) shell was symmetrical.  Having each coil lying outside the other was a great disadvantage as it was not very compact nor was it easy to carry around as the diameter could become very great (some fossil gastropods have been found with shells measuring 8 feet (2.5m!!) in diameter - that's heavy-duty hauling!!!). 

     Then the gastropods underwent a very significant change in their evolutionary path.  This change was the twisting or torsion  that the body underwent.  Most of the body located behind the head, including the visceral mass, mantle and mantle cavity, was twisted 180 degrees counterclockwise (i.e., in a right-handed direction: most species of shell-bearing gastropods are still right - handed, with notable exceptions, such as the Lightning-whelk (Busycon contrarium Conrad, 1840).  Many land and fresh-water species and even some entire genera are also left-handed.) Internally, the digestive tract and nervous system were twisted into a U-shape.  The mantle cavity, gills and renal and anal openings were now located in the anterior part of the body - i.e., just behind the head. 

     The problem of the large unmanageable pretorsion shell was solved with the evolution of the asymmetrical coiling of the shell.  Now, the coils were laid down around a central axis (called a columella and each coil lay beneath the preceding coil.  In order to balance out the weight of this shell, it shifted so that the axis of the spiral slanted upwards and slightly backwards (the asymmetrical  part!).  The shell was now positioned obliquely to the long axis of the body and the gastropod could move about with relative ease. (Diagram) 

     Although there are fossil species showing the pretorsion plano-spiral shell, all existing shell bearing
gastropods have this post torsion, asymmetrical shell. Several problems did arise for the gastropod as a result of this torsion however.  The main one being fouling .  If the water circuit through the mantle cavity had remained as it had been, the anus and nephridia would have dumped directly on top of the head - a dangerous and not too nutritious situation!  Sanitation was thus a great a problem with this new shell design, and the gastropods followed three different courses to solve this problem:  In one group, this problem was solved by the formation, over the mantle cavity, of a cleft or split in the shell and mantle.  At the same time, the anus drew back from the edge of the mantle cavity and moved to a new position just beneath the inner margin of this cleft.  The inhalent current continued to enter over the head and pass over the gills, but now instead of making a U-turn, the water current flowed up and out through the cleft in the shell taking with it the anal and nephridia wastes.  Some of today s gastropods have retained this primitive cleft or shell slit.

     Others modified their internal organ arrangement to have a single-gill arrangement in a new mantle cavity, and developed an inhalant water circuit to the side of the head. .Still another group underwent detorsion , in which the twisting process was reversed and the mantle cavity and anus once again opened posteriorly.  A good example would be the common garden slug. 

     Another consequence of this torsion and new shell position was that it restricted the mantle cavity to one side of the body, and the opposite side of the body was now pressed up against the shell.  This compression resulted in a decrease in size, or the complete loss of, the gill, auricle and kidney on that side of the body. 

     So it came to be that about 500 million years ago, during the Cambrian period, three basically different stocks, with different body-plans arose.  Although most of today s gastropods bear a single, asymmetrically coiled shell, some, such as limpets and abalone have a flat saucer like shell.  Still others have no shell at all as is found in the sea and land slugs.  These shell changes and body adaptations resulted in the Gastropoda being divided into three subclasses: 

     The typical shell of the gastropod is the familiar conical spire composed of tubular whorls. This shell, which is created, maintained, colored and modified by the mantle, contains the visceral mass of the animal: i.e., all its internal organs.  Starting at the apex, the smallest and oldest part of the shell, whorls get successively larger and are coiled about a central axis called the columella, which may be open or closed.  The largest whorl terminates at the aperture or opening where the head and foot of the animal protrude. 

     A shell may be spiraled clockwise (right-handed shell) or anti clockwise (left-handed shell).  When a shell is held so that the apex (top) is up and the aperture facing the person, those with the aperture facing to the right are right-handed or dextral and those that open to the left are left-handed or sinistral.  Both sinistral and dextral shell can be found amongst members of the some species. (Photo 1, Photo 2). (A good article: "Reverse Coiled Gastropods": by the Jacksonville Shell Club on this subject)

     The first shell whorls laid down by the larval gastropod (i.e., while it is in its egg), are called the protoconch  (proto = before, conch is shell).  It is represented by the smallest few whorls at the apex of the shell, and is usually smooth, and lacks many of the characteristics of the adult shell, often being colorless, or of a different color from the rest of the shell. 

     A typical gastropod shell is composed of three layers; the outer periostracum, the middle prismatic layer and an inner nacerous layer.  The periostracum is thin and composed of a horny organic (made out of protein, actually!) material called conchiolin, which is semi-transparent, being a brown color: the thicker the periostracum, the darker the color, and the more the shell underneath is both protected from sand grains and other abrasive elements of the animal's environment, as well as from acidic water, which some of the hardier gastropods and bivalves can survive in. Shell collectors often dissolve the periostracum of their shells, so they can see the beautiful colors and patterns better.  Scientists, however, leave the periostracum on, since it is an important part of the shell.  The two inner layers are composed of calcium carbonate.  In the middle layer (which we normally think of as the outside of the shell, since it contains the colors and patterns, the calcium carbonate is laid down as vertical crystals.  In the thin, inner, nacerous layer, the calcium carbonate is laid down in thin horizontal sheet.  Quite often, there are two or more sheets, each of which reflect light differently, creating the shimmering effect called iridescence  (actually a product of refraction patterns - ask your physics teacher to explain!)  Reserve calcium carbonate is stored in certain cells of the digestive glad and is used for shell repair or to add new growth thus enlarging the shell for the growing animal.  Molluscs can only form shells when they can extract CaCO3 (calcium carbonate) from the water, and keep it from being dissolved again.  Thus, if a lake or stream is acidic, or the soil is acidic (as in Coniferous woodlands), shells, and therefore the animals that make them to protect and support themselves, cannot survive.  Also, below a certain depth in the ocean (which varies with temperature and mineral content, calcium carbonate cannot be deposited, since the water is under-saturated with Ca CO3.  This is called the calcium carbonate compensation depth , and no shell-bearing molluscs can survive below this level.  To summarize, shells can only be formed in fresh waters that are non-acidic, and in the ocean at depths above the level where the water becomes under saturated with Calcium Carbonate. 

     Gastropods show an infinite variety of colours, patterns, shapes and sculpturing of their shell (which is why people collect shells, as opposed to the entire mollusc!!). In some gastropods, the shell is only conspicuously coiled in the juvenile stages.  The coiled nature disappears with growth, and the adult shell represents a single large expanded whorl.  Examples of this are found amongst the abalones (Haliotis), limpets (several families, including Lotiidae and Acmaeidae) and slipper shells (the familiar Crepiduland Capulus).  (The limpets became secondarily symmetrical during in their evolution.) 

     In the family of Vermetidae (the worm shells), the larval and juvenile shells are typical, but as the animal grows older the whorls become completely separate.  The adults look much like a corkscrew, and sometimes don't coil at all, forming a tangled mass of tubes called a colony 

     Amongst many gastropods, the shell has become much reduced or is absent completely.  In other cases the foot and mantle are very large and the mantle has reflexed backwards over the shell so that it becomes totally covered.   These animals are no longer able to pull their bodies completely into their shells. 

     The pulmonates show varying degrees of shell reduction and loss, culminating in the slugs (land and sea), which have no shells at all. 

     The opisthobranchs have a much reduced shell which is closely related to the degree of detorsion they have undergone. A few have well developed shells, such as the bubbles, but most have a much-reduced shell that is often covered by the mantle as is found in the sea hares. 

     Gastropods are able to withdraw into their shells by means of a retractor muscle.  This muscle, called the columellar muscle, arises from the foot and it is inserted into the columella.  The most ancient gastropods, the abalones and limpets have two of these muscles but in the more modern gastropods the left muscle has disappeared.

     The typical foot of a gastropod is a large flat creeping sole similar to the foot design of the ancestral mollusc.  It has become adapted for locomotion over a variety of surfaces. 

     The limpets have become quite well adapted to clinging tenaciously to the hard substratums (rocks, wood, other molluscs' shells, etc.) where they live. Many marine and fresh water gastropods have adapted to living on the soft sandy or muddy bottom.  Others live on seaweed or terrestrial vegetation or under rotting leaves and logs. 

     Typically, a pedal mucous gland opens onto the dorsal or ventral  (i.e., top or bottom) surface of the foot.  This secretes a slime trail over which the animal glides.  Waves of fine muscular contractions that sweep from the anterior to the posterior (i.e., from the front to the back) of the foot provide the power for locomotion. 

     The foot of many gastropods bears either a horny periostracum or calcium carbonate disc, called the operculum.  This structure is found on the posterior dorsal  (the back bottom) portion of the foot.  This is the operculum, and it acts as a trap door  that the animal can pull shut to close off the mouth of its shell, thus protecting its soft body parts, which are safely inside.  The operculum may also be closed tight to guard against dehydration, if it should become necessary. (As in during dry periods or winter (which in many parts of the world is just a dry season), or when a pond dries up!) 

     Some gastropods, such as the marsh-dwelling pulmonates (Melampus, for example, which can be found by looking at the high-tide mark of a salt marsh, usually on or near the salt-marsh grasses, which are called Spartina), extend the anterior portion of the foot and then pull up the rest of their body behind it. 

     In another marine snail (Lacuna), the foot is divided into a right and left half by a groove extending down the middle of their foot.  This snail moves by advancing one side of the foot then the other side (a bit like walking!) 

     The pelagic gastropods, the Heteropoda and the sea butterflies, have adapted to a swimming life style. Being pelagic, the foot has become modified into a powerful finlike, swimming apparatus - in many species, almost looking like wings for flying in the water (hence sea butterflies !) 

     Some of the prosobranch have adapted for burrowing into the soft sand or mud where they live.  Here, the front of the foot called the propodium, acts like a shovel.  It has also developed a dorsal flap-like fold of the foot that acts as a protective shield for the head.  This mode of living is more common in the Pelecypoda (bivalves), however. 

     A few gastropods are sessile (i.e., once they settle down somewhere, they don't move at all (remind you of any couch potatoes you know??)).  They usually attach themselves to the shells of other living or dead molluscs.   The foot has adapted to become a sucker-like. The Worm shells are totally immobile and are either attached to other molluscs or entangled in sponges.

     Gastropods have developed many methods of attaining oxygen from the water or land habitats where they live.  To some extent the exposed body surface, especially that of the mantle, plays a varying role in the respiration of all the gastropods.  Most breathe by means of a gill/s (ctenidia) or secondary gill structures. 

      In the prosobranchs with cleft shells (slit shell, Scissurellidae (which are like mini-slit shells!), and Fissurellidae (the Key-hole limpets) the most primitive type of gill structure and water circulation occurs.  In these gastropods there are two primitive gills and the rectum and anus open beneath the shell perforation or cleft, some distance away from the mouth. 

     In the abalone (Haliotis), the shell contains a row of perforations.  The mantle is split along this line of holes.  Inhalent water is pulled into the anterior portion of the body by the action of the lateral cilia (tiny hairs) on the gills.  The outstretched gills divide the mantle cavity into a ventral inhalent chamber and a dorsal exhalent chamber.  Water flows into the inhalent chamber, passes through the gills, then into the exhalent chamber and finally exits through the shell perforations - an ingenious arrangement, actually! 

     The Scissurellidae have a similar system but instead of having shell perforations, the exhalent current passes out through the long, narrow notch at the posterior mantle edge where the anus exits. 

     Keyhole limpets (Fissurellidae) have conical shells that either have a hole at the apex or a cleft at the anterior margin (i.e., the front end).  The mantle extends through this opening or slit, forming a siphon through which the inhalent water is sucked.  This inhalent water then passes over the gill and exits at the posterior edge of the shell opening (so, it is sucked in the top or the front, through a hole of slit, passes over the gills, then exits out the back - it just sounds so much more scientific when said the other way? (NOTE: Actually scientists don't use technical jargon just to impress or confuse non-scientists (although it sometimes seems that way!!), but because each branch of science has developed a more exact, or precise vocabulary than we use in everyday speaking: the work anterior , for example, has only one meaning, which is precisely defined and cannot be confused with any other word - the word front , on the other hand, could either mean the head area, or the part of the animal that is going forward - which might not always be the head area!  So, it is better in a scientific context or situation, to use the word anterior : than front , because it only has one meaning, while front has at least two!  Half the trick to science is learning how to translate from jargon or science-speak , to ordinary language - hardly anyone ever truly THINKS in jargoneese!!) 

     True limpets lack the hole or cleft and the mantle have developed an overhang.  This overhang forms a pallial groove on each side of the foot.  The inhalent water enters from the anterior of the shell, splits into two streams flowing into these two grooves.  It then passes through the gills merges into a single stream at the posterior and exits out as a single exhalent stream. 

     The remaining prosobranchs have undergone major gill structure and water circulation modifications.  The entire gill axis is attached to the inner, or body side, of the mantle cavity.  They have only a single left gill with filaments that are formed on one side of the axis.  Water enters the mantle cavity to the left of the head and exits on the right side.  To prevent fouling of the inhalent water, the rectum has become elongated and the anus exits near the right mantle edge in the region of the exhalent water current. 

     Many prosobranchs have improved on this system by developing a spout-like inhalent siphon formed by a folding of the mantle s edges.  Some gastropods even carry this a step further and the anterior edge of the shell has evolved to become a grooved, elongated extension to house this siphonal canal.  (A good example of this is found amongst the Tibia shells that have carried this to an extreme in some instances.)

     Some prosobranchs have left the water environment entirely.  These are the operculate land snails, and are not true pulmonates.  They have evolved a lung  from the mantle cavity, as have the pulmonates.  Most of these prosobranchs are restricted to living in moist tropical environments in order to maintain their fluid balance. 

     In the opisthobranchs where partial detorsion (untwisting) has occurred, there has been a loss of the original gill structure and a secondary gill has evolved.  In the nudibranchs and sea slugs where complete detorsion has occurred, the mantle cavity and gill have disappeared all together.  Respiration takes place through the general body surface or through a secondary gill (i.e., a structure which is not really a true gill, but which performs the same function.). To help increase the body surface for this absorption, some have developed numerous projections called cerata.  These cerata are usually arranged in rows.  Not all opisthobranchs have these cerata however. Their cerata often contain such brilliant colours as red, yellow, orange blue and green, which makes them incredibly beautiful! [link]. Some slugs are smooth while others have developed secondary gills arranged in a circle around the anus.  Sea slugs and nudibranchs are amongst the most attractive molluscs one could ever see. 

     In the Pulmonates, the gill has disappeared totally and the mantle cavity has modified to become a highly vascularized (i.e., it has lots of blood vessels to absorb O2 and give up CO2) primitive lung.  The mantle edges fit tight against the foot, except for a small opening on the right side called the pneumostome.  Respiration occurs when mantle floor (acting like a diaphragm) tightens and flattens.  This causes the mantle cavity volume to increase, which in turn sucks air in through the pneumostome. As the air pressure increases the pneumostome closes, to hold the air in.  The muscles of the mantle then relax and arch upwards which increases the gas pressures in this cavity.  This increased pressure facilitates (i.e. assists or makes easier) the absorption of oxygen into the highly vascularized mantle wall chambers and also causes the air to be forced out. 

     Many fresh-water snails are actually terrestrial Pulmonates that have returned to the aquatic environment. Some must return to the water surface to breathe, while others have developed a long retractable siphon from their mantle that they use much like a snorkel (!).  Others can absorb oxygen directly through the mantle surface from the water they draw into their mantle cavity.  In still another group, the mantle cavity became much reduced and a conical projection from the foot called the pseudobranch developed as a secondary gill.

     The gastropods have an open circulatory system - the basic circulatory system found in most molluscs.  They have a hemocoel, or open cavity, into which the blood (called hemolymph , for some reason) is pumped.  Oxygenated hemolymph is collected from the gills or mantle cavity and pumped into a number of open sinuses.  Here the tissues and organs are literally bathed in this oxygen-rich blood.  As it passes over the tissues in these sinuses, it flows into the ctendial (gill) vessels where gas exchange takes place.  It is then again drawn into the atria to be pumped out of the heart. (Diagram)

     The gastropod heart is located anteriorly (i.e., closer to the head) in the visceral mass.  In all but the primitive archaeogastropoda, the right auricle has disappeared or become vestigial due to the loss of the right gill - one of the results of torsion, as discussed above.  The ventricle gives rise to a single, short aorta, which then branches posteriorly to provide the visceral mass with blood, and anteriorly to supply the head and foot.  An enlargement in the anterior vessel - a sort of second heart , functions in controlling blood pressure. Blood from the kidneys usually enters the brachial circulation, but in some cases it returns directly to the heart. In a few Gastropods, such as the family Planorbidae (which you aren't likely to encounter, by the way), the plasma contains hemoglobin instead of hemocyanin. (Hemoglobin uses Iron for transporting Oxygen and Carbon dioxide, while hemocyanin uses copper.  Thus, the blood of most molluscs is a light greenish-blue, instead of the red we usually associate with blood, which comes from the iron in hemoglobin).

     In gastropods, the nervous system is distinctly ganglionated  (i.e., it has well-defined and specialized nerve cells) and is somewhat complex.  It is quite asymmetrical, and twisted into a figure eight as a result of torsion. 

     A pair of cerebral ganglia (which function as a small brain) give rise to nerves anteriorly that connect to the eyes, tentacle and a pair of buccal ganglia.  The buccal ganglia innervate (send nerves to) the muscles of the radula and adjacent structures. 

     A nerve cord extends ventrally from the cerebral ganglion (located on each side of the esophagus) and gives rise to the two pedal nerve cords.  These two cords extend to the midline of the foot to another pair of ganglion, which in turn innervates the foot muscles. 

     Another pair of nerve cords issue from the cerebral ganglia also.  These are the visceral (the viscera are the body organs) nerves, and they travel posteriorly (i.e., away from the head) until they finally meet in a pair of visceral ganglia.  Between the cerebral ganglia and the visceral ganglia and along the visceral nerve cords lay two more sets of ganglia
(NOTE: so, instead of having one large, complicated brain like we do, gastropods have 8 tiny, very simple brains, which coordinate between themselves, although each pair have specialized functions.)  Firstly are the pleural ganglia, which innervate the columellar muscle and the mantle.  The pleural and pedal  (pedal means foot. remember) ganglia are then joined together by means of a pair of connective nerve fibers.  Secondly and more posteriorly are the parietal ganglia.  These innervate the gills, osphridia, and the mantle.  They send out nerve fibers to the various structures of the viscera as well). 

     All gastropods display some degree of ganglial concentration. For example the pleural and cerebral ganglia are always adjacent to each other.  In many cases the visceral ganglia have been fused together to form a single nerve center.  In the genus Haliotis (Abalones), the pedal and pleural ganglia have become fused, and send a long pedal nerve to the foot.  In the Busycon (which include the famous left-handed Lightning Whelk), all except the visceral ganglia have migrated forward and are located around the esophagus and just below the cerebral ganglia.  Here all ganglia connectives have been lost except those between the parietal and visceral ganglia. 

     In the pulmonates, even the visceral ganglia have migrated forward, which has resulted in a secondary bilateral symmetry of the nervous system. 

     Opisthobranchs that have undergone complete detorsion, possess nervous systems that have become symmetrical again, in simple bilateral fashion.

     Gastropods possess the following sense organs:  eyes, tentacles, osphridia (olfactory organs), and statocysts (organs of equilibrium, like our inner ear: they help the mollusc tell which direction is up  - not always easy in water when you aren't on the bottom, or sometimes even when you are). 

• Eyes:  Most gastropods have eyes located at the base of cephalic tentacles.  Most are very simple open pits containing only photoreceptor  (photo means light) and pigment cells.  In the more advanced gastropods the pit has closed over and evolved to contain a proper cornea and lens. The most highly developed eyes of all the gastropods are found in the pelagic sea hares, however most gastropoda eyes are but simple light detection organs.  (Diagram)

• Tentacles:  Prosobranchs possess a single pair of cephalic (on the head, i.e.) tentacles.  Pulmonates and Opisthobranchs have two pair.  These tentacles, in addition to bearing the eyes, contain tactile (i.e., touch) and chemoreceptor cells.  In pulmonates the second pair of tentacles are knobbed.  In nudibranchs, the distal (more distant, as opposed to proximal, or closer) half of the tentacle wall displays plate-like folds called rhinopores .  These knobs and folds increase the surface area for chemoreception. 

• Osphridia:  The evolution of the osphradium (See note) closely parallels that of the gills (NOTE: Latin is tricky, but in science one has to put up with it, since many scientific terms are from Latin, which was once-upon-a-time the language scientists all over the Western World (Europe and the Middle East) used to communicate.  English is now taking that role, although it has not become as fully universal as Latin once was.) .  An osphradium is available for each gill present.  The osphradium has become either filamentous or folded to increase its surface area.  The leading theory amongst scientists is that the function of the osphradium is to detect sediments in the water passing over their gills, but nobody really knows for certain!! 

• Statocysts:  These organs of balance are generally located in the foot near the pedal ganglia, however, some of the opisthobranchs (especially the nudibranchs), these have migrated forward to a position next to the cerebral ganglia 

     Gastropods exhibit virtually every type of feeding possible.   Members of this class can be herbivores, carnivores, scavengers, ciliary feeders, or parasites.  Despite these feeding differences, some generalizations can be made: 

• Gastropods almost always employ a radula (Diagram) in feeding and in many cases this has become a highly developed feeding organ.  The radula may contain as few as 16 teeth  (with only a couple of them fully functional, as in Cones and Turridae, which use them as poison-tipped spears .)  Or as many as 750,000 teeth arranged in rows.  The action of this radula may be that of a grater, a rasp, a brush or a comb, or spear.
• Digestion is always at least partially extracellular in gastropods - there are no gastropod species where all digestion takes place inside the individual cells.
• The enzymes needed for extracellular digestion are usually produced by the salivary glands, esophageal pouches, and/or the digestive gland, or by a combination of all of these structures.  There are few exceptions to this norm.
• The stomach is the site of extracellular digestion and the liver is where absorption and of intracellular digestion takes place, in species where this occurs.
  
• Food is passed through the gastropod gut, or at least partly, by the action of the ciliary tracts.
  
• As a result of torsion, the stomach has been rotated 180 degrees so that the esophagus enters the stomach posteriorly and the intestine leaves it anteriorly (the opposite of most animals!).  In some of the higher Gastropods, the esophagus has started to migrate forward again.
• The enzymes needed for extracellular digestion are usually produced by a) the salivary glands, b) esophageal pouches, and/or c) the digestive gland, or by a combination of all of these structures.

     The most primitive digestive system is found in the Archeogastropoda: Keyhole limpets feed primarily on sponges that are rasped from the substratum by the radula. The salivary glands in these limpets only secrete mucus, which is used for radular lubrication and food transport.  The esophageal pouches are well developed and they as well as the digestive gland produce the enzymes necessary for extracellular digestion. </