The Porifera
Porifera, a phylum within the Kingdom Animalia, consists of all species of sponges, about 5,000 total. Sponges are among the simplest animals on the planet, and it is believed that they evolved from the first multi-cellular organisms which eventually evolved into animals. The phylum Porifera represents the only phylum of the Parazoa branch of Kingdom Animalia--those without true tissues, that diverged from the lineage shortly after the evolution of multicellularism from colonial protists.
Immunity
Immunity