Classification of Mollusca

CLASSIFICATION OF MOLLUSCA

Classification can be great fun and not as utterly boring as it sometimes turns out to be. It’s fun if you can understand the need for it and how its done; how it is a passion for some and when you do not want to learn it for an examination! Enjoy it and then you would even be able to score well in an exam. Seriously- no joke being played on you.

To start with, let me tell you the one thing the scientific community who undertake this task { they are called taxonomists. Taxonomy = Science of Classification } and those who use it are all united in- in pulling their hair out of its roots with this fine art and science of Taxonomy. Why? Do you ask? Because it never is static, the taxonomy and the names are every now and then changing. Complicated? Not really. It is an indication of the continuous nature of learning that is going on as we try to understand the magnificent life on this planet.

Just to remind you that all organisms are classified following a hierarchy which is given below. Each hierarchical group is called taxon, ie Phylum is a taxon, class is a taxon etc etc.(plural -Taxa)

Kingdom ->Phylum ->Class ->Order ->Family ->Genus ->Species

Animalia ->Chordata ->Mammalia ->Primates ->Hominidae ->Homo sapiens
( Human beings or Homo sapiens sapiens are classified as given above)

This is an oversimplified version of classification. In between these groups there are other categories such as Sub-class, Suborder, Superfamily & subfamily, Subgenus, and tribes, subspecies and varieties.

But as a beginner it would suffice if you are able to remember the 7 hierarchical categories given above. The other minute details are important if and when you chose to study something scientifically.

In case you have problems remembering the 7 groups in sequence, then you could perhaps remember this sentence, that I remember reading in a biology text book during my school days:

Kings Play Chess On Fine Grain Sand

NOW ON TO MOLLUSCS.

No doubt you would know that all molluscs come under the Phylum Mollusca. They are the second largest group of living organisms and are represented by a humongous number of species, more than a 100,000. Consider the fact that they first made their appearance around 500million years ago ( That time period is referred to as Cambrian), it’ s no wonder there are so many of them;occupying a diverse habitat – from land to water The variety reaching its magnificent diversity and beauty in the oceans. How many global warming situations and climate changes they must have been witness to! But they have adapted and evolved and now face another challenge with the onset of the latest round of global warming and climate change. Will they survive? To some extent perhaps they will, but their fate lies in the hands of us human beings!

Well to get on with classification.When it comes to sorting and grouping them into categories we can see one long journey of changes from the days of Aristotle to the present.

Aristotle who lived from 384-322 B.C classified Molluscs broadly into Malachia (present day cephalopods) and Ostracodermata.

Then some of these terms underwent changes. Malcahia was changed to Mollia and Ostacodermata to Testacea by Pliny (the Elder) who lived in the years 23-79AD; and finally Mollia was changed to Mollusca by Jonstonus during the 17 th C.

It did not stop there. Classification of molluscs underwent several changes in its hierarchial categories down the years – In 1758 with Linnaeus, in 1795 with Cuvier, in 1806 by Dumeril and from 1801-1819 through Jean Lamarck. All of them modified the classification patterns in some way or other.

These changes became necessary as more and more information about organisms were gathered, learnt. The history of science is fascinating to read. It tells us amongst other things the journey of classification. In the very early years it was exclusively morphology – the external characteristics that dominated classification ; then anatomy-study of internal characteristics came in. As microscopes and X-ray became available valuable knowledge from physiology was used, and today we study organisms at the minutest levels the level of molecules of genes, DNA and proteins. Today increasingly classification at the species level is driven by the genetic composition of the organisms.

Doubtless the classification that you will learn today would undergo changes as more and more genetic information becomes available. Even the species names would change as we begin to understand their genetic mechanisms and their behavioral characteristics. The data for classification today come from diverse fields of biology from ecology to genetics, sophisticated chemical techniques coupled with evolutionary data as well as time tested facts gathered from morphology and anatomy.

BUT DON’T FEEL DAUNTED BY ALL THIS ! Learn classification to use it as a tool to understand and protect an organism and its habitat. The classification of molluscs given here is seen in that perspective. To enable you to know the mollusc you are likely to find (and if you perceive them to be as fascinating as I have found them to be, then) learn a little more to be able to conserve its habitat for it to flourish. There is nothing more fascinating than the architecture of their shells. So start just there and if you find them fascinating then you can learn more details.

You are likely to come across a whole lot of specialized words in describing the molluscs. Look up the glossary for their meanings.

Classifying molluscs :

1. Based on the structure of Radula
2. Based on shell structure

Here is a commonly used classification for molluscs. Phylum Mollusca is classified into seven classes;

1. GASTROPODA (single shelled  cowries, cones etc)
2. BIVALVIA ( two shelled like clams, mussels etc)
3. APLACOPHORA (solenogasters)
4. MONOPLACOPHORA (segmented limpets)
5. POLYPLACOPHORA ( or Amphineura as it was earlier called- Chitons)
6. SCAPHOPODA (tusk shells)
7. CEPHALOPODA (nautilus, squids, Cuttelfish, octopus etc)

The classes are further subdivided into sub-classes, order, family and finally the genus and species. At each level detailed characteristics of the organisms are added in order to find its exact identification. At present I am not giving details of the sub-class or order but am restricting myself to a brief description of each class.

When you open the images of the shells, there would be a brief description of each of them along with the classification. In time to come this page will be updated with a more detailed classification.

GASTROPODA :

This word is a combination of two Greek words : ‘Gastir which means ‘stomach’ and podi which means ‘foot’. So Gastropods can be called ‘stomach-footed’ molluscs!! Funny isn’t it? Try acting out a gastropod!!! As the name suggests these have a well developed muscular foot, sometimes quite large that is used for crawling. There is a head with well developed eyes and tentacles. ( observe a land snail and you can see the tentacles coming out and going in). Except the slugs all gastropods produce an outer shell which is spirally wound. A kind of spiral coiling is seen as the animal grows. The immature organism is bilaterally symmetrical but acquires an asymmetrical shape as it matures into an adult. The coiling of the visceral mass around the central axis occurs through a process called torsion. This class with 17 orders and many families contain animals whose shell shape and structure also help to identify them. A Cone shell is easily distinguished from a top shell or a cowry. Refer to the illustration for parts of this shell. They live on land, freshwater and marine environment.

BIVALVIA :

Bi means ‘two’ and as the name suggests these molluscs possess tow shells. They are also known as Pelecypoda meaning ‘hatchet-footed’. They are bilaterally symmetrical and show no torsion. Head is greatly reduced, no tentacles but have a foot that can often be seen sticking out of the valves. The two valves are joined by hinges and other structures that help the valves to open and close. Refer to illustration for parts of the shell. They are completely aquatic. Eg. Oysters, clams etc

APLACOPHORA :

‘Without plates’ is what this term means. Aplacophorans are small, cylindrical, worm-like creatures that were till 1987 classified under Phylum Echinodermata as Holothurians. These benthic (deep water), exclusively marine molluscs do not have shells but have small calcareous spicules embedded in their mantle. There is no head or specialized excretory organ(nephridia). The foot and mantle cavity are reduced. Sea cucumber is the most familiar example of this class.

MONOPLACOPHORA:

The name means ‘with one plate’ (a single shell). Thought to be extinct, there was a single species discovered in 1952 at Costa Rica and subsequently described in 1957.Along with Neopilina sp . there are about tow dozen species found. They are small and have a single cap like shells that make them look like limpets and hence the common name segmented limpets. Head is reduced with no tentacles or eyes; they have a rounded foot.Many of the organs like the gills, nephridia etc are paired. Another common name to describe them is ‘gastroverm’

POLYPLACOPHORA

‘Bearing many plates’ is the meaning of the term Polyplacophora. The name comes from the characteristic presence of 8plates called valves that are overlapping and attached to a part of the mantle called girdle. These plates are distributed around and underneath the edges. The mouth and the anus are present at the opposite ends of a bilaterally symmetrical body. Eyes and tentacles are absent. Example – Chitons

SCAPHOPODA

Scaphopoda means ‘boat-footed’. This class includes marine molluscs that usually remain buried in snad. The worm-like animal is found within tusk shaped shells that are open at both ends. Thus they are commonly referred to as tusk shells. The animals do not have eyes, tentacles or gills.

CEPHALOPODA
The name means ‘head-footed’, these are exclusively marine molluscs that have a prominent and large head, eyes and tentacles. The eyes of these molluscs are fascinating to study as also some of the mechanisms of camouflage developed by the cuttlefishes. The foot has undergone modification and is seen as a set of arms around the mouth. Shell is either internal or absent, the only exception being species of sub-class Nautiloidea eg Nautilus.