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Octopus and squids generally do not produce external shell (some squid species can have an internal shell). One cephalopod group, the nautiluses, produces an external spiral shell.

Molluscs present extracellular digestion and a complete digestive system, with mouth and anus.

Arthropods present exoskeleton made of chitin. Echinoderms do not have exoskeleton but they present a calcareous endoskeleton. Some chordates also have an external carapace.

Snails, octopuses, squids and oysters are examples of molluscs.

The body of gastropods is divided into three main portions: head, the visceral mass and foot.

The phylum Mollusca is divided into five main classes: pelecypods, or bivalves (Pelecypoda, or Bivalvia), includes oysters, clams, mussels; gastropods (Gastropoda), snails, sea slugs; cephalopods (Cephalopoda), squids, octopuses; scaphopods (Scaphopoda), tooth shells; Polyplacophora, chitons. There are a few other mollusc classes.

Molluscs can be found in the sea, freshwater and in terrestrial environments. 

Because molluscs have a soft body they are more fragile. They also have more difficulty to support their bodies in terrestrial environments or to fixate to substrates in aquatic habitats. Many species solve these problems by secreting a calcareous carapace, or shell, an exoskeleton to support and protect them and to prevent dehydration.

The word “mollusc” means “soft thing”. Molluscs have soft bodies and this feature explains the name of the phylum.

In molluscs there is not a body divided into segments. Considering just this feature it could be said that evolutionarily they are more proximal to nematodes than to annelids.

Molluscs are important players in several food chains in ecosystems. Many marine molluscs are part of a common human diet, like octopus and squid, very appreciated in the East, and oysters and mussels, consumed all around the world. Besides molluscs that are part of the food industry, pearls made by oysters have high commercial value. 

Molluscs have well-developed sensory structures. It is accepted that cephalopods, like octopus and squid, have eyes with image formation. Snails have antennae and a pair of welldefined eyes. Bivalves do not present eyes but they have photosensitive and tactile cells.

Cephalization is evident in molluscs and neurons concentrate in a ganglial pattern.

Pearls are made from small strange particles that deposit between the shell and the mantle of the oyster. These particles trigger a defense process by the organism and they are gradually covered by calcium carbonate layers secreted by the oyster thus giving birth to pearls.

In the artificial production of pearls a small fragment of shell covered with mantle pieces is inserted between the shell and the mantle of an oyster and a pearl is formed around the graft. 

The mollusc foot has the function of locomotion, support, fixation, digging in the environment and sometimes of holding prey.

The terminations of the names given to the main mollusc classes come from the Greek word “podos” that means foot. Gastropods have feet in their ventral region (as the name indicates); pelecypods have ax-shaped feet (ax in Greek is “pelekys”); in cephalopods the feet are near the head.

Some molluscs, like bivalves and gastropods, secrete an external calcareous carapace (the shell). Calcium carbonate is secreted by the mantle, a fold in the epidermis, forming the shell.

In pelecypods, the shell is made of two valves that articulate and contain the individual body within. In gastropods the shell contains only part of the body, the visceral mass.

Some molluscs have a tongue-like structure with harsh saliences similar to small teeth. This structure is called radula. The radula projects outside and helps to scrape food.

Molluscs, with the exception of cephalopods, have an open circulatory system, i.e., the blood circulates within vessels, from the heart, but it also fills open lacunas or cavities. In annelids the circulatory system is closed, i.e., the blood circulates only inside blood vessels.

In cephalopods the circulatory system is closed and this provides more speed and pressure for the blood circulation allowing the existence of species with large bodies, like octopuses and giant squids.

Aquatic molluscs oxygenate their blood through gills in direct contact with water. In terrestrial molluscs the rich vascularity under the mantle cavity absorbs air doing the role of a primitive lung.

Molluscs have one or two pair of spongelike nephridia, similar to kidneys.