Molluscs (Mollusca) are a group of invertebrates that includes squid, octopuses, cuttlefish, nudibranchs, snails, slugs, limpets, sea hares, mussels, clams, oysters, scallops, as well as many less well-known animals. Scientists estimate that there are more than 100,000 species of molluscs alive today. This makes them the second largest phylum of animals, having fewer species than only the arthropods.
Molluscs have soft bodies that consist of three basic parts: a foot, a visceral mass and a mantle. Many species also have a protective shell made of chitin, proteins and calcium carbonate. Since molluscs are so varied in form, it is difficult to use a single representative species to make many generalizations about the group's common anatomical structures. Instead, textbooks often describe a hypothetical "mollusc" that exhibits the features common to many species.
This hypothetical mollusc has a mantle, shell, foot and visceral mass. The mantle is a layer of tissue that covers the visceral mass and in many molluscs it contains glands which secrete a hard shell.
The foot is muscular structure located on the underside of the body. The mollusc secretes mucus from the bottom of its foot which lubricates the underlying surface. This helps the mollusc move, a task accomplished by repeated contraction and stretching of the foot muscle.
The visceral mass, located above the foot and below the mantle, contains the digestive system, the heart, and other internal organs. The circulatory system is open and most species use a single pair of gills to breathe, although some species have one gill while the pulmonates (terrestrial slugs and snails) have rudimentary lungs.
Molluscs transport oxygen throughout their body using a different molecule than vertebrates. Molluscs use haemocyanin, a copper-based molecule while vertebrates use haemoglobin, an iron-based molecule. Since haemocyanin is less efficient at transporting oxygen than haemoglobin is, molluscs tire more easily than vertebrates do. That's why molluscs are more apt to move in quick bursts but cannot sustain their action for long periods of time. The simple fact that molluscs use haemocyanin instead of haemoglobin to transport oxygen throughout their body may have prevented them from dominating marine environments the way modern vertebrates do.
The majority of marine molluscs begin their life as ciliated, free-swimming larvae that later develop into adult form. Freshwater and terrestrial snails develop within the egg and emerge as tiny but fully-formed versions of the adult form. Molluscs are most diverse in marine habitats but also inhabit freshwater and terrestrial habitats.
Molluscs are thought to have evolved from a segmented, wormlike animal similar to present-day flatworms. Their closest living relatives are either the annelids (also known as segmented worms) or platyhelminths (non-segmented worms or flatworms).
Molluscs are divided into the following groups:
- Caudofoveates and solanogastrates (Aplacophorans) - There are about 70 species of caudofoveates and 250 species of solanogastrates alive today. Members of this group are worm-like molluscs that lack a shell and are covered with tiny calcareous spicules.
- Chitins (Polyplacophorans) - There are about 600 species of Polyplacophorans alive today. Members of this group resemble flat slugs that have a series of calcareous plates covering the upper surface of their body. Most polyplacophorns inhabit intertidal zones where they adhere to rocky surfaces to graze.
- Monoplacophorans - The members of this group were thought to have been extinct until 1952 when zoologists discovered 11 living species. Monoplacophorans live in the deep-sea and are somewhat limpet-like in appearance, though they have a number of unique physical attributes that set them apart from all other molluscs, such as six (or seven) pairs of kidneys.
- Tusk shells (Scaphopods) - There are about 350 species of scaphopods. Members of this group have a long cylindrical shell that narrows towards the tip and is open at both ends. Their head is located towards the broader end of the shell and they have numerous tentacles which protrude from the shell opening.
- Bivalves (Bivalvia) - Bivalves are among the more diverse groups of molluscs with 9200 species alive today. Bivalves are notable for their two, mirror-image shell halves (also called valves). Bivalves include clams, oysters, mussels and scallops. They inhabit marine and freshwater habitats.
- Gastropods (Gastropoda) - There are between 60,000 and 80,000 species of gastropods alive today. Gastropods are the most diverse of the mollusc groups. Members of this group include limpets, top shells, periwinkles, sundial shells, sea hares, nudibranchs, snails and slugs.
- Cephalopods (Cephalopoda) - There are about 800 species of cephalopods alive today. Members of this group include squids, octopuses and natiluses.