Phylum Ctenophora

Ctenophora (Greek ktenos, comb; phoros, bearing) are exclusively marine, diploblastic animals with tissue-level organisation and radial symmetry (more precisely, biradial symmetry — see section 2.1.3). Although diploblastic like the cnidarians, their mesoglea is structurally different, containing amoebocytes and smooth muscle cells rather than being purely gel-like.

Their defining feature is eight external rows of ciliated comb plates running along the body, which beat to drive locomotion — the source of their common name, comb jellies or sea walnuts. Many ctenophores show marked bioluminescence, the ability to emit their own light. Unlike cnidarians, they lack nematocysts entirely; instead they capture prey using specialised sticky cells called lasso cells (colloblasts). Digestion is both extracellular and intracellular. …