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 systems.
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The characteristics are described below-
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.
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.
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.
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.
The characteristics of kingdom monera are listed below-
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.
Examples and Habitat
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.
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.
Examples and Habitat
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.
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 and Habitat
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.
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.
Examples and Habitat
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.
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.
Examples and Habitat
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.
It includes soft body tissues usually enclosed in a shell, a head, a foot, a mass of viscera, a mantle that secretes shells in the shelled species and a radula for feeding.
Depending on the structures and life cycles, molluscs are divided into Gastropoda, Bivalvia, Cephalopoda, Polyplacophora, and Scaphopoda.
Molluscs form the basis of fisheries and aquaculture industry particularly clams and oysters since some of them produce pearls, and also useful in biomedical sciences, because of their complicated nervous systems.
Molluscs respond through growth modifications which include shell diversification and feed specialisation which comprise filter feeding, herbivores, and predators as well as burrowing, ability to camouflage and tolerability to extreme climatic conditions.
They consist of the preservation of the species' natural environment, counting populations, breeding in captivity and raising awareness to counter factors that threaten the existence of endangered species and thus total bio-diversity.
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