Flexi Says: The phylum Porifera contains the earliest invertebrates: the sponges. There are about 8,550 living sponge species, which scientists group into four classes: Demospongiae (the largest and most diverse group, containing 90 percent of all living sponges), Hexactinellida (glass sponges), Calcarea (calcareous sponges), and Homoscleromorpha (the rarest and simplest class, only recently recognized). Examples of sponge species include the giant barrel sponge, the red encrusting sponge, and Venus' flower basket.
