Porifera, Cnidaria and Ctenophora

Porifera (sponges) are the simplest animals, mostly marine, asymmetrical, at the cellular level. Their body is full of pores and canals. Water enters through tiny pores (ostia) into a central cavity, the spongocoel, and leaves through a large opening, the osculum. The canals are lined with collar cells (choanocytes) that keep water moving and trap food. Food is digested inside cells. The body is supported by spicules or spongin fibres. Each sponge makes both eggs and sperm (hermaphrodite); they also reproduce by breaking into pieces, and the young pass through a larval stage. Sycon, Spongilla, Euspongia (the bath sponge).

Coelenterata (Cnidaria) live in water, are radially symmetrical, diploblastic, at the tissue level. Their name comes from cnidoblasts, stinging cells with nematocysts, used to hold on, defend and catch prey. They have a gastro-vascular cavity with one opening, the mouth, on a raised part called the hypostome. Digestion happens both outside and inside cells.

Cnidarians come in two body forms: the polyp, which stays attached like a tube (Hydra, Adamsia), and the medusa, umbrella-shaped and free-swimming (Aurelia, a jellyfish). In Obelia the polyp produces medusae and the medusae produce polyps: this alternation is metagenesis. Corals build skeletons of calcium carbonate. Other examples: Physalia (Portuguese man-of-war), Pennatula (sea-pen), Gorgonia (sea-fan), Meandrina (brain coral).

Ctenophora (sea walnuts, comb jellies) are marine, radial and diploblastic, like cnidarians, but they swim with eight rows of ciliated comb plates. Many glow in the dark (bioluminescence). They are hermaphrodite and reproduce only sexually. Pleurobrachia, Ctenoplana.