Phylum Mollusca: Classification, Characteristics, Examples

Phylum Mollusca is the second-largest animal phylum in the kingdom Animalia, after Arthropoda, and the largest marine phylum.

• Animals in this phylum have soft/smooth bodies and a hard exterior or internal shell.
• There are several molluscs species that live in both freshwater and terrestrial habitats.
• Species vary greatly depending on their environment and lifestyle.
• Because of its vast diversity, this phylum is separated into seven classes to make it easier to understand.
• This phylum is distinguished by the presence of a radula, a nervous system structure, and a cavity for breathing and excretion.

Key Terms: Kingdom Animalia, Radula, Phylum Mollusca, Invertebrates, Hemocyanin, Pearls, Myogenic, Visceral Hump, Muscle cells, Marine species

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What is Phylum Mollusca?

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Phylum Mollusca animals have soft bodies that are triploblastic, bilaterally symmetric, and coelomate. Malacology is the study of mollusca. They are sluggish invertebrates with a thin fleshy envelope or mantle that protects the internal organs.

• The name Mollusca was taken from the term given by Aristotle to cuttlefish. Mollusc means soft.
• These species can be found both on land and in the deep oceans. Their sizes range from small to 20-metre-long.
• They have a significant impact on the lives of humans. They are a source of both jewellary and food.
• Natural pearls develop within these mollusca.
• Bivalve mollusca serve as bioindicators in both marine and freshwater habitats.
• Only a few of them, such as snails and slugs, are pests. They have eight arms and no shell.
• Their blood is blue or green due to the presence of a pigment called hemocyanin. Their heart is myogenic.

Phylum Mollusca

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Body Structure of Mollusca

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The body structure of Mollusca consists of 

• Head
• Muscular foot, and
• Visceral hump

Head

It is also known as the "cephalic region." The head comprises a mouth and sensory tentacles. The mouth has a file-like rasping organ known as the radula. Radula helps with feeding.

Muscular Foot

The use of statocysts helps in the mobility and balance of these organisms. 

Visceral Hump

It is found below the mantle, a soft and spongy layer of skin. The mantle is made up of epithelial cells that protect molluscs and muscle cells that help them move by using the force of the water. The region between the mantle and the hump is known as the mantle cavity, and it contains feather-like gills. These provide both respiratory and excretory functions.

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Characteristics of Mollusca

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The following are the characteristics of Mollusca

• Molluscans are found in fresh and saltwater. The few terrestrial molluscans are found in moist soil.
• The body of molluscans has a cavity.
• The body of molluscans can be divided into the head, visceral mass, muscular foot, and mantle.
• Molluscans generally exhibit an organ system level of organization.
• The body of molluscans is covered by a calcareous shell.
• The head of molluscans consists of tentacles and compound eyes.
• The muscular feet of molluscans help in locomotion.
• Molluscans have a well-developed digestive system. The rasping organ for feeding is the radula.
• Molluscans respire through the general body surface – gills or pulmonary sacs.
• Molluscans have an open circulatory system.
• Excretion of molluscans is done through a pair of metanephridia.
• The nervous system of molluscans consists of several paired ganglia and nerves.
• The sensory organs of molluscans are tentacles, eyes, osphradium, and statocysts.
• Some molluscans are hermaphrodites but the sexes are separate in most of them. Fertilization in molluscans may be external or internal.
• Molluscans in general are oviparous with indirect development.

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Classification of Mollusca

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Phylum Mollusca is a very diverse group. They mainly contain marine species with a dramatic range of forms. They can be divided into the following groups.

Aplacophora or Solenogasters

The characteristics of Aplacophora Solenogasters are mentioned below-

• Their body is mainly cylindrical or bilaterally symmetrical.
• They do not have a head, shell, mantel, nephridia, or foot.
• They do not have digestive ceca.
• Their body is covered by spicule-bearing cuticles.
• They contain a dorsal longitudinal keel or crest.
• Examples are Neomenia and Chaetoderma.

Monoplacophora

The characteristics of Monoplacophora are mentioned below-

• Their body is bilaterally symmetrical.
• Their head does not have eyes and tentacles.
• They respirate through externally located gills.
• Their waste is excreted through nephridia.
• An example is Neopilina.

Polyplacophora

The characteristics of Polyplacophora are mentioned below-

• Their body is flattened like a leaf and is bilaterally symmetrical.
• Their shell has eight longitudinal plates.
• Their radula is well developed.
• Their ventral foot has a flat structure.
• Examples are Chiton and Cryptochiton.

Gastropoda

The characteristics of Gastropoda are mentioned below-

• They are found on land and both fresh and saltwater.
• Their head has tentacles, eyes, and mouth.
• They have a spiral-shaped shell.
• They have flat and large feet.
• Examples are Haliotis and Pila.

Scaphopoda

The characteristics of Scaphopoda are mentioned below-

• They are found mostly in marine environments.
• They do not have eyes or tentacles.
• They have reduced feet.
• Their bodies are bilaterally symmetrical.

Pelecypoda

The characteristics of Pelecypoda are mentioned below-

• They reside mostly in aquatic habitats.
• Their bodies are bilaterally symmetrical and compressed laterally.
• They do not have a distinct head.
• They burrow in mud and sand.
• Examples- Mussels.

Cephalopoda

The characteristics of Cephalopods are mentioned below-

• They are found in marine environments.
• Their shell may be external or internal. Some may not have a shell.
• They have separate sexes.
• They go through direct development.
• Examples are Octopus and Spirulina.

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Economic Importance of Phylum Mollusca

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The economic importance of Phylum Mollusca is mentioned below

• Teredo, also known as the shipworm, causes severe damage to wooden structures steeped in the sea.
• Mollusks are a great source of food in many different parts of the world, namely China, Japan, Malaya, Europe, and America.
• Shells of freshwater mussels are used in the pearl button industry in all parts of the world.
• Oyster shells are mixed with tar to create roads in America. Lime from these shells is used in the poultry industry for the formulation of eggshells.
• Most ecologically valuable pearls are created by pearl oysters Pinctada Margaritifera and Pinctada Mertens. They are found on the coasts of China, India, Sri Lanka, Japan, and in the warmer parts of the Indian and Pacific Oceans.

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Examples of Mollusca

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Some examples of animals belonging to Phylum Mollusca are:

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

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Things to Remember

• Phylum Mollusca contains invertebrate animals with soft bodies that are triploblastic and bilaterally symmetrical and coelomate.
• Molluscans are a source of jewelry as well as food. Some of them also are a source of natural pearls.
• The body of molluscans can be divided into the head, visceral mass, muscular foot, and mantle.
• The body of molluscans has a cavity.
• The body of molluscans is covered by a calcareous shell.
• The head of molluscans consists of tentacles and compound eyes.
• The muscular feet of molluscans help in locomotion.
• Phylum Mollusca can be divided into the following groups - Aplacophora or Solenogasters, Monoplacophora, Polyplacophora, Gastropoda, Scaphopoda, Pelecypoda, and Cephalopoda.

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