Most molluscs, are about 1 to 20 cm (0.4 to 8 in) long, and some are scarcely visible, but the largest is the giant squid weighing up to 2 tonnes. They are also the largest invertebrates.

The encyclopedia Encarta says that earlier estimates of the number of mollusc species sometimes exceeded 100,000, but more recently this figure has been reduced to about 50,000.

Mollusca or molluscs are shell fish that include snails, slugs, cockles, oysters, chitons, limpets, clams, mussels, nudibranchs,(sea-slugs), tusk shells, cuttlefish, octopus, squid, and paua.

Molluscs are important to us because for lots of them we are able to eat, and many make beautiful collections.

Although most molluscs have a shell as adults, the octopus, squid, and deep-sea forms do not. They do, however, have a small, shell-like structure, called a shell gland, which is present for a short time during embryonic development.

Molluscs have soft bodies but like crustaceans they have a external skeleton to protect and support the body. These shells have lots of different patterns, colours, and shapes (the reason they are collected).

Some molluscs like nudibranches have no shells and some have internal ones such as the tiger cowrie. This uses a mantle to cover and protect the shell, this makes it smooth and shiny. The mantle also acts as a camouflage.

Others called bivalves have two hard hinged stream lined shells held together with a strong muscle these being mussels,cockles, oysters and scallops. And some have a tongue that is used for digging into the sand such as cockles, pipies, and toheroa. These bivalves feed on plankton.

The rings found on the outer surface of the clam shell are growth rings and are useful in determining the clam’s age.

Members of the class Scaphopoda (tusk shells) have a long, tapered, slightly curved shell and live on sandy bottoms.

Members of the class Gastropoda (snails and slugs) are asymmetrical and have only one shell or, as in slugs, no shell at all.

The molluscs of the class Cephalopoda, which include octopus, cuttlefish, and squid,are without a foot and shell, but have the development of arms around the mouth.

The scallops are able to move around by leaping, this is achived by the opening and shutting their shells and forcing out water like a jet much the same as the squid.

And yet some with hard shells such as limpets paua and oysters clamp on to rocks. These molluscs the limpets and paua are both herbivores. Some cone shells have long teeth much like a harpoon and stab their prey these being worms and some small fish.

Carnivorous molluscs have fewer and larger teeth than the herbivores and can feed on sea weed and other dead organic matter.

Most molluscs have poor vision, however cephalopods such as squid have eyes with lenses, retinas, and other features remarkably similar to those of vertebrates. Predators may be detected by a well-developed sense of smell.

Some molluscs, such as octopus, cuttlefish, and squid, exhibit a complicated courtship behavior. And also possess considerable ability to learn from experience. Reproduction

The basic mollusc pattern is to have separate sexes, with sperm and eggs spawned into the water, where fertilization and early development occur.