Mollusca is a large Phylum of Invertebrate Animals. They are also known as Molluscs or Mollusks. Molluscs are highly diverse, not only in size and in anatomical structure, but also in behaviour and in habitat. Mollusca includes familiar organisms like Snails, Octopuses, Squids, Clams, Scallops, Oysters and Chitons. The Gastropods (snails and slugs) are the most classified Species, accounting for 80% of the total Mollusca. There are around 85,000 recognized extant Species of Molluscs.
Mollusca like Gastropods, Cephalopods and Bivalves appeared in the Cambrian period 542 to 488.3 million years ago. Molluscs are a important source of food, jewellery, tools and even pets to humans throughout the history. Certain Molluscs like Snails and Slugs are serious agricultural pests and make up a major component of fouling communities both on docks and on the hulls of ships. Some like Water Snails are hosts of disease, known as Snail Fever (schistosomiasis, bilharzia, bilharziosis). Various Species of Molluscs are Endangered and Threatened like the Seychelles Molluscs, Biomphalaria tchadiensis, Alycaeus balingensis etc.
There are variations regarding the classification of Molluscs. Some authors combine the Caudofoveata and Solenogasters into one class, the Aplacophora.
Below is a common Classification of Molluscs with examples.
Caudofoveata - Example Worm-Like organisms consisting of 120 Species.
Aplacophora - Example Solenogasters consisting of 200 Species.
Polyplacophora - Example Chitons consisting of 1000 Species.
Monoplacophora - Example Limpet-like organisms consisting of 31 Species.
Gastropoda - Example Sea Butterfly, Snails, Slugs etc. consisting of 70,000 Species.
Cephalopoda - Example Squid, Octopus, Cuttlefish, Nautilus consisting of 900 Species.
Bivalvia - Example Clams, Oysters, Scallops etc. consisting of 20,000 Species.
Scaphopoda - Example Tusk Shells consisting of 500 Species.
Rostroconchia - Extinct, probable ancestors of Bivalves.
Helcionelloida - Extinct, Snail-like organisms such as Latouchella.
Mollusca Characteristics
Bilaterally Symmetrical - Mollusca have a balanced distribution of duplicate body parts or shapes.
Soft Unsegmented Body - Molluscs have a body which is not divided into segments and is soft.
Body Wall - All mollusks possess a flexible body wall, which surrounds a body cavity containing the internal organs. The wall, which varies greatly in shape in different Species, is usually folded to form a structure called the Mantle (also known as a pallium), which is attached at the top of the body and surrounds it like a tent; the shell is formed on the outside of the Mantle. On the underside of the body the wall is usually stretched out to form a thickened mass called the foot. The wall is covered by an outer epidermis and an underlying dermis. The epidermis usually contains gland cells that secrete mucus, which in mollusks has a variety of important uses, such as locomotion, food entrapment, and prevention of water loss. Muscle tissue is found in the body wall, and is particularly plentiful in the foot, which is used for locomotion in most mollusks (although some swim and some are sedentary), and in the mantle in Species with reduced shells.
Mollusca Shell - Molluscs, in general, produce an external skeleton (Exoskeleton) which is called a Shell, which is composed of a limey material: calcium carbonate (CaCO3) The shell serves both protective, and supportive purposes. The one feature common to all Molluscs is the presence of a fleshy Mantle. This is a fold or lobe (or a pair of them) of fleshy material, which secretes, modifies and lines the shell. Shell is often common in in the shells of Snails and Clams or the plates of Chitons.
Digestive Tract - The digestive tract of the Mollusca is complex. The foregut region consists of an oesophagus and a mouth cavity, which contains a toothed belt called the 'Radula', found in almost all Mollusks and peculiar to the Phylum. The Radula is usually used for scraping food, such as algae, from surfaces. The number and form of Radula teeth are highly variable; some Species have a single Radula tooth while others may have several hundred thousand. In some the teeth are hollow and poison-containing and are used as weapons; other Radula modifications exist. The stomachs of Mollusks are generally complex, and these, too, differ with the Species and according to the feeding habits of the Animal.
Mollusca Respiration - Respiration amongst Molluscs is conducted through gills called Ctenidia (ctenidium), located in the mantle cavity (the space between the mantle and the body wall proper) and varies with the Species and with the type of habitat. For example, intertidal marine Mollusks are exposed to air and water alternately and must be able to respire in both conditions; terrestrial Species have lost their Ctenidia, replacing them with lungs that can function in both water and air. Excretion of wastes is done through structures called Metanephridia and through the body and gill surfaces.
Circulatory and Nervous Systems - Mollusc blood circulates through the gill filaments, where exchange of carbon dioxide and oxygen occurs between the blood and the water flowing over the gill surface. Most Molluskan blood contains a respiratory pigment called Hemocyanin, a copper compound. When oxygenated such blood is bluish in colour; when de-oxygenated the blood is colourless. Only a few Mollusks have Haemoglobin in their blood. Blood circulation is variable within the Phylum but is generally mediated by a muscular heart, which distributes the blood to the tissues. Most Mollusks possess well-developed sensory organs. The highest degree of development of the nervous system is found in the class Cephalopoda (octopuses, squids, and nautiluses).
Moisture Dependant - To survive all Molluscs must have moisture. To stay alive, they must keep their soft bodies moist at all times and for some like those which live in hot dry deserts environments, this is done by curling up in their shell, secreting a mucous plug and staying holed up until the next bit of moisture comes along. This is also the reason why majority of Mollusca are found in marine environment.
Weight - The 'Largest Mollusca' weighs around 270 kg and the 'Smallest Mollusca' weighs around a milligram or less. Size - The 'Largest Mollusca' measures up to 12 metres long and the 'Smallest Mollusca' is a millimetre or less in length.
Mollusca Distribuion(geographical range & habitat)
Molluscs occur in almost every habitat found on Earth, where they are often the most conspicuous organisms. While most are found in the marine environment, extending from the intertidal to the deepest oceans, several major Gastropod clades (biological group) live predominantly in freshwater or terrestrial habitats.
Diet - Molluscs eat a variety of food material. Molluscs may be Herbivores, Carnivores, Scavengers, Deposit Feeders, Filter Feeders or Parasites. Common Garden Snails and Slugs feed on plant matter, while Octopuses and Squids eat Crustaceans, Fish and other Mollusca.
Predators - Mollusca Specialists when it comes to Predators, are Snail Kites, Starfish, Otter, Raccoons, Muskrats etc. A variety of other Birds, Mammals etc are also Predators of Molluscs.
History and Evolution of Mollusca
The Late Cambrian fossil Plectronoceras is considered to be the earliest clearly Cephalopod fossil, as its shell had Septa and a Siphuncle, a strand of tissue that Nautilus uses to remove water from compartments that it has vacated as it grows, and which is also visible in fossil ammonite shells. However, Plectronoceras and other early Cephalopods crept along the sea floor instead of swimming, as their shells contained a 'Ballast' of stony deposits on what is thought to be the underside and had stripes and blotches on what is thought to be the upper surface. All Cephalopods with external shells except the Nautiloids became extinct by the end of the Cretaceous period 65 million years ago.However, the shell-less Coleoidea (squid, octopus, cuttlefish) are abundant in modern times. The Early Cambrian fossils 'Fordilla' and 'Pojetaia' are regarded as Bivalves. 'Modern-looking' Bivalves appeared in the Ordovician period, 488 to 443 million years ago. One Bivalve group, the 'Rudists', became major reef-builders in the Cretaceous, but became extinct in the Cretaceous-Tertiary extinction. Even so, Bivalves remain abundant and diverse.
Mollusca Reproduction
Molluscs of simple anatomy rely on external fertilization. All produce eggs, from which may emerge 'Trochophore Larvae', more complex 'Veliger Larvae', or 'Miniature Adults'. Two gonads sit next to the coelom, a small cavity that surrounds the heart and shed ova or sperm into the coelom, from which the nephridia (organ similar to kidney) extract them and emit them into the mantle cavity. Molluscs that use such a system remain of one sex all their lives and rely on external fertilization. Some Molluscs use internal fertilization and/or are Hermaphrodites, functioning as both sexes; both of these methods require more complex reproductive systems. Trochophore is Planktonic and feeds on floating food particles by using the two bands of cilia round its equator to sweep food into the mouth, which uses more cilia to drive them into the stomach, which uses further cilia to expel undigested remains through the anus.
