Relationships at the base of the Metazoa. Cnidarians are amongst the simplest animals at the tissue grade of organization, and are often regarded as the closest outgroup to the Bilateria. Within the Cnidaria, the class Anthozoa is basal, whereas the Hydrozoa is derived. The sponges (Porifera) are unquestionably animals, but represent a lower level of organization. The affinities and relationships of genera mentioned in the text are indicated.