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The Phylum Mollusca
(an Introduction, for High-school level students, and all others interested!!)

By Avril Bourquin
Science Editing by Ross Mayhew
May, 2000


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All of the Taxonomy in the Advanced Article is being updated for in regards to structure and picture content. Thank you for your patience



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.

Turris babylonia
(Linne, 1758 )
Picture courtesy of Ross Mayhew

Miscellaneous Conidae
Picture coutesy of Paul Monfils

     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.


The Molluscs we recognize today are divided into seven divisions called "classes"
Let us take a very brief look at these classes now:
  • The POLYPLACOPHORA contain about 900 living species and are commonly known as chitons.  They have a shell consisting of eight, usually overlapping plates, held together by a leathery "girdle".   The animal is bilaterally symmetrical. With a well-developed foot surrounded by a groove in which there are 6 to 88 pairs of gills.

Tonicella insignis (Reeve, 1847)

The head lacks eyes and tentacles, but usually has light-sensitive areas and chemical receptors,for finding food and heading in the proper direction! All chitons are marine inhabitants and most make a living by grazing algae from rocks and other hard substrates. The great majority of them dwell in shallow and intertidal waters, but a few occur in depths down to 5,000+ meters. They range in length from 3 to 400+ millimeters. (1/8" to 1ft 4"+).

  • The APLACOPHORA consist of about 250 living species (perhaps more, as they are relatively poorly studied!) of marine, wormlike, bilaterally symmetrical animals living at moderate, to very great depths, usually on or in soft bottoms.  They have no shell, but have calcareous spicules in the body surface.  The foot is restricted to an anterior pedal shield or to a narrow groove running the length of the body. Aplacophorans have a radula and a posterior mantle cavity.  Some are detritus feeders, others are predators.  They range in length from 1 to 300 millimeters. (If you read and go along with the views held by Brusca & Brusca (see bibliography) you will also include the Class of Caudofoveata & Solenogastres.  These are two very small classes that many scientists combine to form the Class Aplacophora. 

The Caudofoveata are aberrant molluscs that lack shells. They are quite common in the deep sea buried in the soft sediments.  The Solenogastres also lack shells, are also found in very deep water and generally live on the surface of the substratum.)

  • The SCAPHOPODA comprise about 350 living species. Commonly known as "tusk shells", they are bilaterally symmetrical and their elongate, tubular, tapering shells are open at both ends. The conical foot can be protruded for use in burrowing only.  The head is rudimentary and lacks eyes and tentacles. It feeds by contractile filaments called captacula, which are withdrawn into the body cavity when they meet up with food.  All scaphopoda are marine, and live buried in muddy or sandy bottoms, where they feed on detritus.  They range from 2 to 150 millimeters (i.e., up to 6").

Dentalium (Dentalium) elephantinum

(Linnaeus, )1758
  • The CEPHALOPODA contain about 600 to 650 living species.  This class includes octopus, squid, cuttlefish and nautilus.  They are bilaterally symmetrical and often highly streamlined. Tentacles surround the head, and a funnel coming from the mantle produces jet propulsion.  Only a few cephalopoda produce a calcareous shell.  They have an advanced nervous system and are the most intelligent (See Octopuses are Smart Suckers article) of all the invertebrates.  All are marine inhabitants and are predators or scavengers.  They range in size from 10 millimeters to 20 meters for the giant squid.




Chambered Nautilus
  • The GASTROPODA  is by far the most successful class of all the molluscs, with at least 60,000 living species.  Gastropods generally have a single-valved shell, which is usually spiraled; however, this is absent in the slugs and semi-slugs.  They have a head with cephalic tentacles and a well-developed foot used in crawling. Some gastropods have lungs for respiration, others gills.

Early in their larval stage of development, the visceral mass and mantle cavity rotate up to 180 degrees counterclockwise; in a process know as torsion. (This does not occur in some of the slugs though.)  This brings their organs from a posterior position to an anterior position behind their head. In most cases, the soft animal is able to retract into their shells for protection.  Some gastropods also have an operculum (trap door) connected to their foot that they can pull in after their soft body parts sealing off their shell from the environmental hazards or as protection from enemies.  Gastropods are very successful in marine, freshwater and terrestrial habitats.  Their size is from 0.5 to 750 millimeters. (i.e., up to 2 1/2 feet long)

  • The BIVALVA, or PELECYPODA (the Bivalves) comprise about 10,000 living species.  They have two valves made of calcium carbonate (in a hard form called "aragonite"), connected by a flexible ligament and an "adductor muscle" for closing the valves tightly.

The mantle cavity is enlarged, enclosing the visceral mass and other internal organs.  There is no differentiated head or cephalic region, and the radula common to all other molluscs is absent.  Most are filter feeders, with the gill acting as a food collecting and sorting organ, in addition to filling its respiratory function.  The mouth usually has a pair of labial palps on either side that handle and direct the food collected by their gills. Bivalves inhabit all of the world's marine and freshwater habitats which have a pH greater than about 5 - any more acidic than that, and they can't form a shell fast enough to prevent it from being dissolved again! They range in size from 0.5 millimeters to almost 1.4 meters (that's 1400mm, or 4'8"!!)

  • The MONOPLACOPHORA are mostly are known by their fossil records; however, there are about a dozen living species today.  They have several foot retractor muscles, gills, and hearts similar to those of the annelid worms; however, their bodies are not segmented.  All Monoplacophorans are marine inhabitants grazing on algae and microorganisms on the hard ocean bottom.  They live at depths of 200 to 6,000 meters and they range in size from 2 to 35 millimeters.

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:

     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 Aplacophorans and the Polyplacophorans, 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.


Anatomy and Physiology of the Classes








References Used


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