p. 244. Basal animals: sponges, corals, and jellyfish
- Peter Holland
‘Basal animals: sponges, corals, and jellyfish’ describes the four non-bilaterian phyla — Porifera (sponges), Cnidaria (sea anemones, corals, and jellyfish), Ctenophora (comb jellies), and the Placozoa — that separated from other animal groups early in evolution. The key characteristics of each phylum and their subdivisions are outlined. Sponges and placozoans lack precise symmetry, while comb jellies have ‘biradial’ symmetry, meaning that their bodies are symmetrical by a 180-degree rotation. The fourth and largest of the non-bilaterian phyla, the Cnidaria, contains animals whose bodies also lack head-to-tail, top-to-bottom, and left-to-right axes, and with few exceptions also have radial or rotational symmetry.