Phylum Mollusca: Definition, Characteristics, Classification, Exampales

Phylum Mollusca is a heterogeneous group of invertebrates that includes the most common examples such as snails, clams, squids, and octopuses. These are studied under conchology-which is the science of mollusc shells. The soft body in molluscs is usually enclosed in a calcareous shell. In molluscs, the muscular foot is usually used for movement. There are species of molluscs wherein the mantle secretes the shell and a radula for feeding. This is one of the important topics from the Animal Kingdom chapter of Biology.

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This Story also Contains

1. What are molluscs?
2. General Characteristics of Mollusca
3. Classification of Mollusca
4. Mollusca Examples

What are molluscs?

Mollusca belongs to the largest group of invertebrates and most numerous families contain such representatives as snails, clams, squids, and octopuses. Molluscs are very crucial in ecosystems as they form part of food chains support biofiltration, and are sources of income through the sale of fish, shellfish and other aquatic livestock.

The history of the knowledge of the mollusc’s existence can be traced back to the dawn of the naturalists who documented the various types and habitats of the molluscs. Studies have established their physiological structure-activity and role in the ecosystem and thus their value in the evolution of species, particularly in aquatic ecosystems.

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General Characteristics of Mollusca

The characteristics are described below-

Soft-bodied Animals

Mollusca which are soft-bodied animals are largely enclosed in a shell formed from calcium carbonate though some groups such as the cephalopods, including octopuses, and gastropods, including slugs, do not possess shells.

Body Structure: Head, Foot, And Visceral Mass

The body of a mollusc is divided into three main parts the head, which is involved in the reception of stimuli through the sensory organs as well as feeding in some species through the mouth the foot which is a muscular organ used in movement and attachment and the visceral mass, internal organs like the heart, digestive, gill and reproductive systems.

Mantle and Shell

The mantle is another important tissue layer that in shelled molluscs covers the visceral mass and secretes the shell. The shell gives some support and often acts as armour, or is in some way missing in most of the species.

Symmetry and Body Plan

Molluscs show bilateral symmetry which means that the body can be divided into two halves. Its body plan is versatile, and depending on what it must achieve, its forms and functions may significantly differ as it fits the environment they are in.

Classification of Mollusca

The characteristics of kingdom monera are listed below-

Class Gastropoda

Characteristics

Gastropods are often one-spiralled and maybe have shells, they have a feature of torsion in their developmental cycle that twists their visceral mass, and their head is differentiated with sensory tentacles and a radula which is a mouthpart used in feeding.

Some examples are the terrestrial snail known as the garden snail belonging to the Helix aspersa species and the marine snail the queen conch which is Strombus gigas. Gastropods belong to the large class of opisthobranchs, located in fresh and marine water, as well as on the territory of land in various parts of the Earth.

Class Bivalvia (e.g., clams, oysters)

Characteristics

Members of this subclass have a two-chambered shell that is hinged and acts like a box to cover the body. They are without an individual head and radula but they have bilateral gills for filter-feeds and breathing.

In estuarine habitats, there are oysters (Crassostrea virginica) and in the sandy bottoms of marine as well as freshwater locations clams (Mercenaria mercenaria). Bivalves are sessile suspension feeders distributed in different water types all over the world.

Class Cephalopoda (e.g., squids, octopuses)

Characteristics

Molluscs cephalopods are those belonging to the group of animals that have a large head with large eyes and a circle of tentacles or arms around the mouth. Most of them are characterized by having only a rudimentary fused plate or no protection at all (other than nautiloids).

Examples are the common squid which is worldwide and the giant Pacific octopus which is Enteroctopus dofleini which is near coastlines. Cephalopods are fast-moving animals, which inhabit the watery environment from the shores to the ocean abysses.

Class Polyplacophora (e.g., chitons)

Characteristics

Chitons or polyplacophorans have oval-shaped bodies with hard sides and a length of around 2-4 cm and width of 1-2 cm and are covered with eight shelly plates. Their one is wider and is utilized for gripping the rocks and scraping algae from them.

Some of such species include the lined chiton, scientifically known as Tonicella lineata and it is common in intertidal regions across the globe, ensuring itself a firm hold to rocks. They are correct, sometimes called giant grazers, feed on algae and other kinds of marine plants existing in coastal areas.

Class Scaphopoda (e.g., tusk shells)

Characteristics

Scaphopods or tusk shells are those snails which have elongated slim shell that is also hollow and has two openings. It has a smaller head covered with tentacles and a transformed lower appendage which enables the fish to dig into sand or mud.

Some examples include buried shallow marine sediment dentalium shells commonly known as Dentalium entalis. Another mollusc, scaphopods, feeds by filter feeding it moves water through one end of the shell, to catch tiny species. It is widespread in oceans and seas.

Mollusca Examples

Following are a few examples of the phylum Mollusca:

• Turbinella (Shankha)
• Loligo (Squid)
• Limax (Slug)
• Unio (Freshwater mussel)
• Pila (Apple snail)
• Helix (Garden snail)

Octopus (Devilfish)

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