Protozoa are single celled organisms that are very diverse groups. Of the existing protozoa, there are about 21,000 species that occur as free-living in a variety of habitats while another 11,000 species occur as parasitic microbes in both vertebrate and invertebrates hosts.
Invertebrates are multi-cellular animals without a backbone or bony skeleton. This is by far the largest group in the animal kingdom: 97% of all animals are invertebrates. Worldwide in distribution, they include animals as diverse as sea stars, sea urchins, earthworms, sponges, jellyfish, lobsters, crabs, insects, spiders, snails, clams, and squid. Invertebrates are especially important as agricultural pests, parasites, or agents for the transmission of parasitic infections to humans and other vertebrates. Invertebrates serve as food for humans and are key elements in food chains that support birds, fish, and many other vertebrate species.
