The animal kingdom is the first kingdom in the 5-kingdom classification which includes all the animals and this kingdom has a wide range of diversity in it. There are over a million species present among us when we look around. They have different shapes, forms, sizes, and structures. So, biologists have classified them into 5 kingdoms. The 5-kingdom classification was given by R.H. Whittaker in 1969. These 5 kingdoms are Monera, Protista, Fungi, Plantae, and Animalia. These kingdoms consist of many phyla.
So, In this article, we are going to discuss Mollusca which is the second-largest animal phylum of the kingdom Animalia after Arthropoda and the largest marine phylum. Animals in this phylum, are soft-bodied/smooth-bodied with a hard external or internal shell. Species in this phylum are around 85,000. 60,000-1,00,000 fossil species are there of Mollusca. There is a number of molluscs species which are found in freshwater and terrestrial habitat. According to the habitat and behaviour of species, they have a wide range of diversity. Due to this vast diversity, this class is divided into 7 classes which makes it easier for us to understand this phylum. The main distinguishing features of this phylum are the presence of a radula, the structure of the nervous system and the cavity for breathing and excretion.
- Terrestrial (land snails, land slugs) or Aquatic (marine or freshwater) – Clams, Mussels, Oysters, Octopus, Squid, Scallops, etc.
- The level of organization that is seen in this phylum is the Organ-system level organization.
- These are bilaterally symmetrical organisms.
- Molluscs are triploblastic animals i.e., they have ectoderm, mesoderm, and endoderm germ layers.
- The body of most species of molluscs is covered by a calcareous shell, except Octopuses.
- The coelom is present, that is, the internal cavity.
- The body is unsegmented.
- The body is divided into a head, muscular feet, and a visceral hump.
- The head consists of tentacles and compound eyes, which also act as sensory organs.
- For locomotion, they have muscular feet.
- A well-developed digestive system is present.
- They feed with the help of radula.
- The excretion generally takes place by a pair of metanephridia.
- The body surface and gills/pulmonary sacs help in respiration.
- They have separate sexes, but some of them are hermaphrodites also.
- Fertilization may be external or internal.
- Development in this phylum is indirect and they are oviparous in nature.
- Acts as a source of both food and jewellery. Molluscs are popularly known for producing natural pearls.
The body consists of a head, muscular foot, and visceral hump.
- Head: It is also known as the cephalic region. The Head consists of a mouth and sensory tentacles. The mouth has a file-like rasping organ for called radula. Radula helps in feeding.
- Muscular foot: With the help of statocysts, it helps in the movement and maintaining a balance of these species
- Visceral hump: It is present below the mantle, which is a soft and spongy layer of skin. The mantle is composed of epithelial cells for the protection of molluscs and muscle cells which sometimes help the molluscs to move using the force of the water. The space between the mantle and the hump is called the mantle cavity which has feather-like gills. These have respiratory and excretory functions.
This phylum is classified into 7 classes, namely, Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.
Aplacophora or Solenogaster
- Bilateral and cylindrical body shape.
- Primarily found in benthic marine habitats.
- The calcareous shell is absent.
- The epidermis has spicules covering the body.
- They lack head, shell, mantel, nephridia, and foot.
- They have a single cap-like shell enclosing the body.
- They have a looped digestive system, multiple pairs of excretory organs, many gills, and a pair of gonads.
- Gills are externally located.
- The Head lacks eyes and tentacles.
- Commonly known as Chitons.
- Have armour-like 8-plated dorsal shell.
- Bear a flat ventral foot that is adapted for suction to rocks and other substrates.
- The mantle extends beyond the shell in the form of a girdle.
- Have a well-developed radula.
Bivalvia or Pelecypoda
- Found in marine and freshwater environments.
- Bilaterally symmetry and laterally compressed body enclosed by a shell in two hinged parts.
- Most of them are filter feeders and have no head or radula.
- Gills get evolved into Ctenidia, which is a specialized organ for feeding and breathing.
- Usually, bury themselves in sediments on the seabed.
- Examples: clams, oysters, mussels, scallops, etc.
- Also known as Stomach foot.
- These include shell-bearing species as well as species with reduced shells.
- Asymmetric and usually have a coiled shell.
- The Head has a mouth, eyes, and tentacles.
- Well-developed radula for digestion is present.
- They have modified feet used for crawling. At the centre of the foot around the perpendicular axis, visceral masses in shelled species show torsion and this is one of the distinguishing features of this phylum.
- Examples: snails, slugs, conchs, sea hares, sea butterflies, etc.
- These are head-foot animals.
- They are mostly found in the marine environment.
- They display a variety of colourations which are used for camouflage.
- Mostly carnivorous predators are found in this class.
- The circulatory system in this class is closed type.
- Presence of a well-developed nervous system along with eyes.
- Locomotion is facilitated by ejecting a stream of water for propulsion (“jet” propulsion).
- Sometimes the shell is not present at all and sometimes it is present externally or internally.
- Examples: octopuses, squids, cuttlefish, nautilus, etc.
- Also known as tusk shell or tooth shell.
- Absence of eyes.
- These are usually buried in the sand with the anterior opening exposed to water.
- The radula is present.
- The foot is modified into tentacles with a bulbous end, known as Captaculae, which are used to catch and manipulate prey.
Depending on the shell, there are two kinds of molluscs:
- Univalves: The shell is formed by a single piece of an asymmetric arrangement called Torsion Example: Snails and Slugs.
- Bivalves: The shell consists of two valves with a ligament joining them, which helps the mollusc to open and close itself accordingly. Example: Mussels, and clams.
Mollusca reproduces sexually. Most of the species are dioecious and oviparous. They have indirect development, i.e., the embryo hatches from the egg to form a larva, which undergoes metamorphosis to become an adult.
Examples: Octopus (devilfish), Pila (Apple snail), Pinctada (pearl oyster, Sepia (cuttlefish), etc.
FAQs on Mollusca
Question 1: What is the purpose of a radula?
The radula is generally used for cutting or scraping food in the phylum Mollusca. This is sometimes compared to the tongue. The main function of this radula is feeding. But the forces which are used for cutting the food with teeth are unknown. This can be recognized as a rasping organ in molluscs. Scraping of food from rocks and other substances is done by radula.
Question 2: How do Gastropods move?
The visceral mass in the shelled species displays torsion around the perpendicular axis on the centre of the foot which is modified for crawling.
Question 3: What is the purpose of chromatophores?
These are organs that are present in the skin of many cephalopods, such as squids, cuttlefish, octopuses, etc., whose primary function is camouflage. These are the pigment-containing cells whose function is to adjust the body’s colouration to its environment.
Question 4: What are the distinguishing features of Gastropods?
They are asymmetrical and usually have a spiral shell. The visceral mass in the shelled species displays torsion around the perpendicular axis on the centre of the foot is the key characteristic of this group.
Question 5: Explain reproduction in molluscs.
Simple reproductive organs are present in molluscs. They have separate sexes. Parthenogenesis is also found in this phylum in subclass Prosobranchia. The rest mostly reproduction occurs through sexual mode.
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