Molluscs

Snails, slugs, clams, mussels, squids and octopuses are all very different-looking animals. However, they are all molluscs, and they live almost everywhere - on the rocky shore, in freshwater habitats, and in your garden.

Common Mollusc

Generally molluscs have:

an unsegmented, soft body
a muscular foot or tentacles
a mantle that can secrete a shell.

Most, but not all, molluscs have:

an internal or external shell
a radula (tongue with teeth).

After the Arthropods, the Molluscs are one of the largest animal groups with more than 100 000 described species of Mollusc alive today and 35 000 fossil species.

They range in size from just 2 mm to over 20 m in the giant squid, and occupy all major habitats. Molluscs are very ancient organisms believed to have evolved from a flatworm like ancestor during the Precambrium marine environment where they occupied the shore area offering them a great abundance and diversity of food and habitats. (See Kimberella below.)
It is these varied conditions which probably led to their great diversity. Later bivalves and gastropods moved from salt to brackish then fresh water habitats. And eventually some gastropods left the water and invaded the land, but were limited by their need for humidity and calcium.
Despite the many differences in external appearance, their internal structure is very similar. Mollusca means 'soft-bodied' and, although some have developed a tough shell, they are all soft on the inside.
Here's the anatomy of a bivalve:

Anatomy of a Mollusc

Here's a snail:

Anatomy of a Snail

Molluscs are further classified into several major groups. However, the main groups we will look at are

Gastropods (slugs, land snails, marine snails, freshwater snails, limpets),
Bivalves (clams, shipworms, cockles, piddocks),
Cephalopods (squid, octopus, nautilus), and
Chitons.

All molluscs have:

An internal body form containing internal organs, including the digestive tract, paired kidneys, and reproductive organs.
A mantle that surrounds but does not cover entirely the animal and secretes a shell (if one is present). The mantle also contributes to formation of gills or lungs.
A head/foot region containing sensory organs and a muscular structure (foot) used for locomotion.
The foot is a muscular structure used for locomotion, attachment to a substrate, food capture, or a combination of functions.
A radula is an organ that bears many rows of teeth and is used for grazing on food.
The nervous system consists of several ganglia connected by nerve cords.
Most mollusks have an open circulatory system: a heart that pumps hemolymph through vessels into a hemocoel. Blood diffuses back into the heart and is pumped out to the body again. Some mollusks are slow moving, and have with no head, while others are active predators that have a head and sense organs.

Here's an illustration showing the relationship between the various Molluscs and the fossil record.

Evolutionary Relationships of Mollusca

In the above drawing, a composite of several illustrations, you see a hypothetical ancestor

Kimberella

Recently, paleontologists are considering the Late Precambrian fossil, Kimberella, as a candidate for that ancestor.

For a short time, Kimberella was known only from Australia, with the exception of some questionable north Indian specimens.
Scientists now have over thirty-five specimens of this fossil from the Winter Coast of the White Sea in northern Russia. Study of the new material reconstructs Kimberella as a bilaterally symmetrical, benthic (bottom feeder) animal with a non-mineralized, univalved shell, resembling a mollusc in many respects.
This is important evidence for the existence of such animals in the Precambrian and indicates that the origin of the higher groups of mollucs and arthropods (insects, arachnids, crustaceans, centipedes) lies well back in the Precambrian.
Here's an artist's drawing of Kimberella.

Artists drawing of a Kimberella