The Mollusca are one of the largest groups of invertebrate animals, which are similar in biologic organization.
They are marine and only a few bivalves and gastropods can live in fresh water. One group of gastropods (the pulmonates) lives on the land.
The Shell of mollusca may be chitinous or calcareous (calcite or aragonite) and is secreted by the mantle. Most molluscs have an external shell of calcium carbonate, some have an internal shell like Sepia and belemnites, and others donít have shell like Octopus and some gastropods (terrestrial gastropods).
The soft parts of mollusca (visceral mass) include the heart, alimentary tract divided into mouth, esophagus, stomach, an intestine and ends with the anus. There is also digestive glands, kidneys, nervous, circulatory and muscular systems. The anus discharges waste products into a posterior space termed the mantle cavity. This cavity has both an inlet and outlet to the sea, through which waste material discharges and respired water too. Gills are found in the mantle cavity, these gills are used for respiration as well as gathering food particles from the water. They are present in all mollusca except land gastropods (pulmonates) which have lungs.
All mollusca except pelecypods have a head with a mouth containing radula (a belt of teeth for mastication) and sense organs.
Feeding in Mollusca and their habit:
Mollusca are divided into five classes:
The Amphineura are marine molluscs having a shell composed of eight calcareous plates. They are rare as fossils. Ex. G. Chiton.
The Scaphopods are marine molluscs with small tapering shell open at both ends. The anterior wider end is permanently embedded in sediment. The anus is at the upper end. They have a foot used for burrowing. The shell is aragonitic and sometimes longitudinally striated. They are rare as fossils. Ex. G. Dentalium.
Pelecypods are molluscan animals having a calcareous shell, consisting of two valves. The soft parts are enclosed within these two valves. The two valves are articulated along a hinge line, which carries teeth and sockets. They are closed and opened by muscles.The soft parts are enclosed in the mantle, this mantle secrets the shell. The mantle is attached at the dorsal part but free at the ventral part leaving a depression known as pallial line. During life, all pelecypods must have a current of water entering the gills for respiration and for bringing food to the mouth. This current enters the body through a lower inhalent canal. The waste products are carried out through an upper exhalent canal. Some pelecypods have an exhalent tube called siphon; the siphon is drawn within the shell by muscles attached to the inner surface just below the posterior adductor muscles. The places of attachment of muscles form the embayment of the pallial line known as the pallial sinus.
The valves of pelecypods are held together by adductors muscles which pass from one valve to the other. They close when the muscles contract and open when the muscles relax. There is also an elastic structure along the hinge line called ligament that helps in opening the valves. The ligament is external but there is another similar structure helps in opening the shell, this structure is called resilium. The resilium is internal and lies in a depression called resilifer. Some species have both ligament and resilium.
Shells having one adductor muscle are called monomyarian, those having two muscles are called dimyarian, and the two muscles may be equal (isomyarian) or unequal (anisomyarian).
The shell of pelecypods as mentioned before, consists of two valves. The two valves articulate by means of teeth and sockets, which are found in each valve.
The earliest part of the valves is called beak and the rounded part adjacent to the beak is called umbo.
The shell is bilaterally symmetrical with the plane of symmetry passing between the valves.
Structure of the shell:
When studying pelecypod shells in thin sections under microscope, three layers are recognized: -
Chemically, bivalve shells may be calcitic,
aragonite or both together
Q. Compare between Brachiopods and Pelecypods?
Dentition means the arrangement of teeth and sockets along the hinge line. Generally, there are two kinds of teeth:
1. Cardinal teeth: lies beneath the umbons.
2. Lateral teeth: lies on either side of the umbons.
Types of dentition:
1.Taxodont:Teeth are numerous and are arranged along a straight or curved hinge.
Ex. Arca, Barbatia, Glycemeris, Nucula.
2.Dysodont: Teeth are small and simple.
3.Isodont:Teeth are very large and lie on either side of a central ligament pit.
4. Schizodont:Teeth are very large and have many parallel grooves normal to the axis of the
tooth Ex. Trigonia
5. Heterodont: with two or three cardinal teeth below the umbo as well as elongated lateral teeth
anterior and posterior to these. Ex. Venus
6. Pachydont:The teeth are very large, heavy and blunt.
Ex. Rudistids and G. Chama
7. Cryptodont (Desmodont):Neither teeth nor sockets.
Bivalve shell morphology and mode of life:
The shape and general morphology of bivalve shells directly reflects their mode of life. Modern bivalves are grouped into the following morphological categories:
1- Infaunal shallow burrowing. Ex. Lucina, Venus.
2- Infaunal deep burrowing. Ex. Mya
3- Epifaunal attached by byssus. Ex. Mytilus, Tridacna
4- Epifaunal cemented to the rock. Ex. Oysters
5- Free-lying. Ex. Gryphaea
6- Swimming. Ex. Pecten
7- Boring and cavity dwelling. Ex. Lithophaga
Byssus:is an organ composed of chitinous fibers, which attach the pelecypod shell to the substratum.
Oysters are pelecypods, which have two unequal valves, the larger valve is the left valve and is cemented to the substrate. The smaller valve acts as a lid or operculum. They are characterized by having one muscle scar (monomyarian) and absence of teeth and sockets.
During Cretaceous oysters formed banks reaching more than 5 meters in height and extending for several kilometers. They are still living today and live in aggregates cemented together such as in the Gulf of Oma(Khor Fakkan). Ex. Ostrea, Exogyra.
Rudistids are pelecypods, which have two unequal valves. The larger valve is the right valve, it has a conical shape and is fixed to the substrate. The smaller valve is the left o, which acts as a lid.
Rudistids are extinct and died out at the end of the Cretaceous. They are good index fossils for time correlation and biostratigraphic Zonation.
Gastropods are mollusca which have a shell made of a single piece, which is usually sealed apically and coiled helically and lacks serial muscle scars.
They have a well-developed head region with tentacles, eyes and other sense organs. There is a ventral foot for creeping and specialized structures (gills or lungs) for breathing. Fertilization is internal and sexes are united or separate depending on the species.
The mouth contains the radula, which is the feeding organ in gastropods. The radula consists of rows of small teeth to shred food material. There is no radula in bivalves (Pelecypods).
The gastropod shell begins with the protoconch, which consists of several minute whorls. The rest of the shell consists of a coiled cone, open at the larger end where the aperture lies. This cone is made up of several whorls; the line of contact between adjacent whorls is called suture. The suture is absent in shells which donít have the whorls in contact.
All whorls expect the last one constitute the spire, the last whorl constitutes the body whorl. Usually; the whorls are not divided into chambers except in rare cases where the whorl contains many chambers.
In some Gastropods there is only one chamber (uncoiled shells), in others, there are several whorls arranged along one plane (planispiral). In most gastropods, the whorls are coiled spirally in more than one plane, such coiling is called helicoid or trochospiral coiling.
If the whorls are in contact along the inner surface, an axial pillar will be formed in the place of the axis of coiling, this pillar is called columella. If the whorls are not in the contact along the inner surface, there will be a space in the place of the axis of coiling, this space is called umbilicus.
It is an opening in gastropod shell found in the body whorl. In some gastropods the aperture is closed by a horny or calcareous plate called operculum.
The margin of the aperture adjacent to the columella or umbilicus is called inner lip; the opposite margin is called outer lip.
The aperture may be provided with a channel called siphonal canal to allow water from the gills to pass out to the exterior. Apertures with such siphons are called siphonostomatus whereas those without siphons are called holostomatus.
To orient a gastropod shell, hold it so that the apex is up and the aperture faces you.
- If the aperture lies on the right, the shell is dextral.
- If it lies on the left, the shell is sinistral.
Most gastropods have an aragonitic shell. In fossil shells the aragonite either recrystallizes to calcite or may be dissolved.
Most gastropods live in marine water, they also live in fresh water and on land. Land gastropods are called pulmonates because of having lungs instead of gills.
There is an important group of gastropods, which died out at the end of the Cretaceous, this group is called Nerinids. The shells of Nerinids have a long spire and must be studied in thin sections.
Types of coiling in gastropods:
Non-coiled: cap-shaped gastropods without coiling.
Ex. Patella, Fissurella
Planispiral: coiling in which the whorls are arranged along one plan.
Helicoid (Trochospiral): coiling in which the whorls are arranged spirally in more than one
plane. There are different forms of helicoid shells:
Conical (Trochiform): conical shells with acute spire and flat base.
Ex. Trochus, Tectus.
Biconical: the shell appears as if it consists of two cones with a common base.
Ex. Strombus, Conus.
Obconical: similar to biconical but the upper cone (spire) is obtuse.
Turbinate: similar to conical but with convex base.
Turreted: with long acute spire.
Fusiform: spindle-shaped, thickest at the middle and tapering at each end.
Convolute: The last whorl is very large and conceals all or nearly all earlier whorls.
Vermiculate: like worms.
Auriculate: the last whorl is very large with a large aperture.
Cephalopods are mollusca having a bilaterally symmetrical body with a highly developed head surrounded by tentacles.
Most cephalopods have a shell that is coiled planispirally (planispiral coiling). Paleozoic cephalopods have a shell that is a straight, curved or coiled tapering cone. The shell may be external (Nautilus) or internal (Sepia), other cephalopods have no shell (Octopus).
Some cephalopods died out at the end of Paleozoic (those having conical shells), others at the end of Mesozoic (ammonites and belemnites). Nautilus is the only living cephalopod that is similar to fossil forms.
Morphology of cephalopod shell (G. Nautilus):
The shell of Nautilus is planispirally coiled. The animal occupies the terminal living chamber. As the animal grows it secrets a new chamber and shuts off the older one by a septum (septum is a plate that closes the older chambers in Cephalopods). The septa are perforated in the middle to allow the siphon to go through.
Siphon: is a fleshy tube that extends backward to the earlier chambers.
The siphonal tissues secrete a tube of calcium carbonate around it, this tube is called Siphuncle.
As mentioned before, the animal occupies the last chamber, which is called body chamber. The other chambers are called gas chambers because they are filled with gas similar to air but with more nitrogen and less oxygen. This gas increases the buoyancy of the shell, making it easier to swim.
All chambers except the last one are together forming the phragmocone. The last (body) chamber is seldom preserved in fossil state.
The animal protrudes outward through an opening called aperture. Some Cephalopods close their aperture by a single or double plate when the animal withdraws into the shell; this plate is calcareous and is called aptychus (double) or anaptychus (Single).
Types of Cephalopod shells:
Orthocone:straight, conical shell
Cyrtocone: curved shell
Gyrocone: shell coiled in an open spiral and the whorls donít touch each other.
Lituiticone:coiled in the early stage and later uncoiled.
Planispiral: shell coiled in one plane only.
Evolute: all whorls are visible
Involute: the last whorl covers all earlier ones.
Ex. Dactylioceras: Evolute
Trochospiral:shell coiled in more than one plane
Septa and suture in Cephalopods:
Septa are transverse partitions in cephalopod shells that divide it into chambers or camerae. They are convex towards the apex and concave towards the aperture.
The line of contact of septa with the wall of the shell is called suture. The suture may be circular or undulated and is very important in the evolution of Cephalopods. The part of suture that is convex towards the aperture is called saddle, the other part convex towards the apex is called lobe.
There are four types of suture:
- Straight or curved suture line.
- Ex. Orthoceras
- Cambrian to Recent.
- Rounded saddles and angular
- Ex. Goniatites
- Devonian to Permian.
- Rounded saddles and crinkled
- Ex. Ceratites
- Permian to Triassic.
- Both saddles and lobes are
- Ex. All Ammonite shells
- Triassic to Cretaceous.
Classification of Cephalopods:
Cephalopods are divided into three main subclasses
Tetrabranchiate (four gills).
Shell: External, calca.
Straight curved or coiled.
Ex. Nautilus, Orthoceras, Cyrtoceras.
Age: Cambrian to Recent.
This subclass includes extinct cephalopods that died out at the end of Cretaceous.
Shell: External, calcareous (aragonitic)
Straight curved or coiled.
Suture: goniatitic, ceratitic and ammonitic.
Ex. Goniatites, Ceratites, Turrilites, ScaphitesÖ etc.
Age: Devonian to Cretaceous
This subclass includes a group of extinct organismscalled Belemnites, which died out at the
end of Cretaceous.
Dibranchiate (two gills).
Shell: internal, calcareous (Belemnites and Sepia) or no shell as in octopus.
This subclass is further subdivided into order Sepioidea and order Belemnoidea. The latter is
very important index fossils especially in the Jurassic and Cretaceous.
Belemnites are extinct cephalopods whose skeleton (guard) is a bullet-shaped and made up of calcite. They died out at the end of Cretaceous like ammonites.
The shell of Belemnites is internal and composed of three parts:
This is a bullet-shaped cylinder of solid calcite. In cross section the rostrum consists of radially oriented calcite needles.
This is a conical, thin-walled aragonitic structure; it is divided into chambers (camerae) by septa. The septa are perforated by a siphuncle.
This is a blade-like plate consists of calcified conchioline. It extends from the dorsal part of the shell to protect the viscera.