Phylum Mollusca is one of the largest and most diverse groups in the animal kingdom. It includes soft-bodied, mostly unsegmented animals found in marine, freshwater, and terrestrial habitats. Members of this phylum range from simple forms like snails to more complex organisms such as octopuses and squids.
Characteristics of Phylum Mollusca
The following features characterise members of Phylum Mollusca:
- The level of organisation that is seen in this phylum is the Organ-system level organisation.
- 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 for Octopuses.
- The coelom is present in the internal cavity of these organisms.
- 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 a 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.
- Fertilisation 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.
Body Structure of Phylum Mollusca
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 called a radula. Radula helps in feeding.
- Muscular foot: With the help of statocysts, it helps in the movement and maintains the 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.
Classification of Phylum Mollusca
This phylum is classified into 7 classes, namely, Aplacophora, Monoplacophora, Polyplacophora, Bivalvia, Gastropoda, Cephalopoda, and Scaphopoda.
The seven classes of the phylum Mollusca are given below:
1. 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 a head, shell, mantle, nephridia, and foot.
2. Monoplacophora
- 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.
3. Polyplacophora
- Commonly known as Chitons.
- Have an 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.
4. Bivalvia or Pelecypoda
- Found in marine and freshwater environments.
- Bilateral symmetry and a laterally compressed body enclosed by a shell in two hinged parts.
- Most of them are filter feeders and have no head or radula.
- Gills have evolved into Ctenidia, which are a specialised organ for feeding and breathing.
- Usually, bury themselves in sediments on the seabed.
- Examples: clams, oysters, mussels, scallops, etc.
5. Gastropoda
- 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.
6. Cephalopoda
- 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 a 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.
7. Scaphopoda
- 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.
Reproduction of Phylum Mollusca
Mollusca reproduce 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.