Phylum Mollusca: Have you seen snails during rainy seasons? These have a coiled shell on their body and have tiny popped out eyes. These snails belong to the Phylum Mollusca. Phylum Mollusca is the second most abundant phylum in the Animal Kingdom. Phylum Mollusca contains soft body animals usually protected by a calcareous shell and a ventral muscular foot.

Do you know which phylum Octopus belongs to? Molluscs such as oysters, squid and cuttlefish are utilised as food in various countries. The shells of several molluscs are of ornamental value. It is used in pearl extraction. In this article, we have provided detailed information on Phylum Mollusca. Scroll down to continue reading!

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Definition of Phylum Mollusca

The members of the Phylum Mollusca are soft-bodied animals, unsegmented with a distinct head, muscular foot, and visceral lump. The study of Mollusca is known as Malacology. Molluscs are mostly seen in Marine and freshwater. They mostly possess organ system level of organisation.

History of Phylum Mollusca

The name Mollusca came from the Latin, “mollis” meaning “soft” and this term was first used by the French Zoologist Cuvier in \(1798\) to describe squids and cuttlefish, an animal whose shell is reduced internal or entirely absent. It was only later that the true affinities between these species and other molluscs, such as snails and bivalves were fully recognized.

Over \(160000\) species have been described, of which around \(128000\) are living and about \(35000\) are recorded as fossil species. The study of molluscs is known as Malacology and the study of the molluscan shell is known as Conchology.

What are the Characteristics of Phylum Mollusca?

Members of this phylum are commonly called soft-bodied animals since they have a soft body enclosed in a calcareous shell. The characteristics of Phylum Mollusca are listed below:

1. These are mostly marine habitats, and some are freshwater habitats.
2. They are triploblastic, bilaterally symmetrical with organ system level of organization.
3. The body of molluscs is unsegmented with a distinct head, muscular foot and visceral hump.
4. The body is represented by a soft visceral mass that encloses all the internal organs.
5. The visceral mass is covered by a fold of a skin called the mantle. The mantle is a thin, fleshy fold of dorsal body wall covering the body. It encloses a space between itself and the body which is called the mantle cavity.
6. The shell is secreted by the mantle and is made up of calcium. Shell may be external or internal (e.g., squid, cuttlefish) or absent (e.g., Octopus).
7. The coelom is greatly reduced. It is restricted to the pericardial cavity and to small spaces within kidneys and gonads. Spaces amongst the viscera contain blood and form haemocoel.
8. Respiration occurs through a specialized organ called ctenidia but may also occur by the mantle and in semi-terrestrial form like Pila by a pulmonary sac called lungs.
9. Excretory organs are in the form of structures called organs of Bojanus. Ammonia is the chief excretory matter.
10. The digestive tract is complete with a digestive gland or liver. In some molluscs like Pila, the buccal cavity contains a grasping organ, the radula, with transverse rows of teeth for cutting the grass.
11. They have an open type circulatory system. It includes the dorsal heart, arteries that open into sinuses and veins.
12. Blood is usually blue due to the presence of a copper-containing blue respiratory pigment called haemocyanin. Among the molluscs, cuttlefish are exceptional in having a closed circulatory system.
13. The nervous system comprises a paired cerebral, pleural, pedal and visceral ganglion joined by the nerve connectives and commissures.
14. Locomotion is by the locomotory structures called a muscular foot.
15. In many molluscs, eyes (photoreceptors) and tentacles are present on the head.
16. Some molluscs have Osphradium for testing the chemical and physical nature of water.
17. The sexes are separate, and fertilization is internal. They are oviparous. The development is either direct or indirect. When the development is indirect, it includes a characteristic larva, veliger, trochophore or glochidium. Asexual reproduction is absent.

Fig: Snail

Classification of Phylum Mollusca

According to the body symmetry and the characteristics of food, shell, gills, mantle, muscles and radula, molluscs are classified into six classes. The classification followed in this article is by Morton and Yonge \(\left( {1964} \right)\) and they classified Mollusca into six classes as they merged class Aplacophora and Poly-placophora into a single class Amphineura. The classification of the Phylum Mollusca are as follows:

Class 1. Monoplacophora

1. They are bilaterally symmetrical with a dome-shaped mantle.
2. These are marine habitats with internal segmentation.
3. Flattened limpet-shaped shell with spirally coiled protoconch.
4. Five pairs of gills in pallial grooves.
5. The radula is in a radular sac and the intestine is coiled.
6. Heart of two pairs of auricles and a single ventricle.
7. Example: Neopilina galatheae

Class 2. Amphineura

1. These are marine and have an elongated body with a reduced head.
2. These have radula. The foot is ventral, large, flat and muscular.
3. Shell as 8 dorsal plates or as spicules.
4. Non-ganglionated nerve ring around the mouth with two pairs of interconnected nerve cord.
5. External fertilization: trochophore larva.
6. Example: Neomenia, Lepidopleurus, etc.

Class 3. Scaphopoda

1. These are marine habitats.
2. The body is bilaterally symmetrical, elongated and enclosed in a tusk-shell that opens at both ends.
3. They do not possess a head; mouth with tentacles; no eyes.
4. They have a conical foot, the radula is present and gills are absent.
5. Mantle tubular completely enclosing the body.
6. Mouths are surrounded by lobular processes or outgrowths.
7. Heart rudimentary.
8. The kidneys are paired.
9. Sexes are separate (dioecious).
10. Trochophore larva.
11. Examples: Dentalium, Cadulus, Pulsellum.

Class 4. Gastropoda

1. It is the largest class of Mollusca.
2. Mostly they are marine, some are freshwater or terrestrial.
3. The shells are present or may be absent in some; univalve and usually coiled.
4. They have a well-developed head with eyes and tentacles, radula is also present.
5. The foot is large and flat.
6. The digestive system has the muscular pharynx, a long oesophagus, stomach, long coiled intestine, and anteriorly placed anus.
7. Respiration by gills in most forms and through the wall of the mantle cavity in some animals.
8. The open circulatory system and heart is enclosed in a pericardium.
9. Sexes separate, they are dioecious in most molluscs.
10. Examples: Pila, Bulla, Limax, etc.

Class 5. Pelecypoda

1. These are mostly marine and some are freshwater habitats.
2. The body is enclosed in a bivalve shell and laterally compressed.
3. They do not have a head, tentacles, eyes, jaws and radula.
4. Gills or ctenidia are paired, one on each side.
5. Their alimentary canal is coiled with large paired digestive glands.
6. Sexes are separate or united.
7. Mostly filter-feeding.
8. Development is accompanied by metamorphosis which usually includes a trochophore larva.
9. Examples: Ostrea, Mytilus, Unio, etc.

Class 6. Cephalopoda

1. These are marine habitats and-swimming animals.
2. The body is elongated dorsoventrally and bilaterally symmetrical with the head and trunk.
3. Head distinct and large with well-developed eyes and mouth.
4. Example: Octopus.

Examples of Phylum Mollusca

Some of the examples of the phylum Mollusca are as follows:

1. Pila

Classification of Pila–
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Order: Architaenioglossa
Genus: Pila

Fig: Pila

2. Sepia

Classification of Sepia
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Order: Sepiida
Family: SepiidaeGenus:Sepia

Fig: Sepia

3. Octopus

Classification of Octopus–
Kingdom: Animalia
Phylum: Mollusca
Class: Cephalopoda
Order: Octopoda
Genus: Octopus

Fig: Octopus

4. Unio

Classification of Unio
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Unionida
Family: Unionidae
Genus: Unio

Fig: Unio

Economic Importance of Phylum Mollusca

The economic importance of Phylum Mollusca are as follows:

1. Molluscs like oyster, squid and cuttlefish are used as food in many countries.
2. The shell of many molluscs is of ornamental value, i.e., used in pearl extraction.
3. Dentalium is used as a decorative piece.
4. Sepia ink has medicinal value.
5. Shells of oyster are mixed with tar to make roads.

Summary

The phylum Mollusca is the second largest phylum in the Kingdom Animalia. These are soft-bodied animals with an outer covering of calcium shells. Phylum Mollusca’s body is divided into head, visceral mass, and muscular foot. They move with the help of a muscular foot.

The members are marine forms, and some are freshwater forms. Molluscs like oysters, squid and cuttlefish are used as food in many countries. Some of the examples of animals belonging to Phylum Mollusca are Pila, Unio, Octopus, Sepia, etc.

FAQs on Phylum Mollusca

Q.1. What animals belong to the phylum Mollusca?
Ans: The animals that belong to the phylum Mollusca are Pila, Sepia, Octopus, Unio, snail, etc.

Q.2. What is the economic importance of snails?
Ans: They possess anti-cancer properties and boost the immune system due. They are rich in antioxidants, fatty acids, iron, calcium, etc.

Q.3. What are the 6 classes of the phylum Mollusca?
Ans: The 6 classes of the phylum Mollusca are Monoplacophora, Amphineura, Scaphopoda, Gastropoda, Pelecypoda and Cephalopoda.

Q.4. Which phylum is considered the second largest phylum in the animal kingdom?
Ans: Phylum Mollusca is considered the second largest phylum in the animal kingdom.

Q.5. What are the five characteristics of the phylum Mollusca?
Ans: The five characteristics of the phylum Mollusca are as follows:
1. These are mostly marine habitats, and some are freshwater habitats.
2. They are triploblastic, bilaterally symmetrical with organ system level of organization.
3. The body of molluscs is unsegmented with a distinct head, muscular foot, and visceral hump.
4. The body is represented by a soft visceral mass that encloses all the internal organs.
5. Excretory organs are in the form of structures called organs of Bojanus. Ammonia is the chief excretory matter.

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