Over the past 500 million years, mass extinctions have occurred in which at least of the species on earth became extinct.

as resources become scarce because of population growth, the growth decreases after few years due to lack of resources and food.

one characteristic that is shared by all is a soft body. many

of these invertebrate animals also have an outer shell to protect their

body. oysters, clams, snails, and mussels are all mollusks. so are octopuses, squids, and slugs. mollusks live on land and in freshwater.