Mollusca
Mollusca has at least 50,000 species, although it is estimated that there are more than 200,000, such as clams, oysters, snails, and octopuses.
Mollusca are one of the organisms that have an exoskeleton, which is made of chitin, such as the shells of snails. Although they are very diverse in shape, and have, for example, from one foot on a snail to 8 feet on an octopus, they all seem to retain some patters, such as all seem to have an area for their head distinctly separated from the area with their feet.
Mollusca are also major parts of almost every ecosystem, for example in some cases up to 3,000 species have been found in one coral reef. One of the reasons for this is that they have highly variable diets.