a shell is a covering around the body of a shelled animal & covering something is a kind of protection ->  a shell is a kind of protection for some animals  a shell is a kind of protection for some animals ->  the shell of an animal is used to protect the animal  a shell is a part of a snail & a snail is a kind of animal ->  a snail is a kind of animal that has a shell  the shell of an animal is used to protect the animal & a snail is a kind of animal that has a shell -> a shell is a part of a snail for protecting the snail