Animals are a vast and varied lot in the kingdom Animalia, and include creatures as developed as human beings and as different as the vertebrates (birds, reptiles, fish), the Mollusks (like snails and clams), Arthropods (like shrimps, insects and millipedes, etc.) Annelids (like leaches and earthworms), Jellyfish and sponges. Animals differ from other living organisms in many simple ways. Animals belong to a group of multi-cellular organisms whose cells have a nucleus and other structures enclosed within membranes, a feature that sets them apart from bacteria. Absence of rigid cell walls distinguishes animals from plants, fungi and algae. Some animals like insects undergo a metamorphosis or transformation in shape and form. Most animals can move and must take in food for sustenance. They are heterotrophic which means they digest food in an internal chamber. All animals feed on other living organisms and so their classification falls into groups like herbivores, carnivores, omnivores and parasites. Although most animals follow the sexual reproduction route, several animals adopt asexual reproduction through a process called parthenogenesis that provides fertile eggs without mating.