Invertebrates comprise between 95 and 99 percent of all animal species, so any list is bound to be no more than an overview of the group. Invertebrates are divided among eight separate phyla; it is almost impossible to generalize these phyla. Major phyla of invertebrates are Arthropoda, Cnidaria and Porifera.
Arthropoda is the most common and diverse phylum in the animal kingdom. Almost 75 percent of the animal species on Earth are arthropods, including the almost 1 million species of insects. Spiders, scorpions, mites and ticks are all arachnids, which is the other major group of arthropods. Crustaceans are considered arthropods because of their jointed exoskeletons.
Cnidarians are a large, easily defined phylum that is identified by a single cell type unique to its members. All cnidarians have cnida, or stinging cells, and no other animal has them. Cnidaria includes jellyfish, sea anemones and comb jellies, among others.
Poriferans are among the most primitive, in the sense of resembling the common ancestor, of animals. The phylum Porifera includes all sea sponges. Unlike most animals, sponges live out their lives anchored to a single spot on the sea floor. They totally lack organs and other differentiated structures and are so simple they can be broken into their individual cells, left in a dish for several weeks and then reformed into a functioning sponge.