Ctenophores are commonly known as sea walnuts or comb jellies. They are exclusively marine, radially symmetrical, diploblastic organisms with a tissue level of organisation. The most distinctive feature is the presence of eight external rows of ciliated comb plates on the body, which help in locomotion. Digestion in ctenophores is both extracellular and intracellular. Bioluminescence, the property of a living organism to emit light, is well marked in ctenophores. Sexes are not separate, so the animals are hermaphrodite. Reproduction takes place only by sexual means; there is no asexual reproduction. Fertilisation is external, and development is indirect. Examples of this phylum are Pleurobrachia and Ctenoplana. In the broad classification, ctenophores are grouped with cnidarians as radially symmetrical animals with a tissue level of organisation and an incomplete digestive system, but they differ in having comb plates rather than cnidoblasts, in being exclusively marine, and in reproducing only sexually. These contrasts are common in NEET-style questions. Remember that comb plates are the only locomotory organs of the phylum and that the property of bioluminescence is especially well developed here, which makes it a useful identification point for the phylum in the exam.