Q.Bioluminescence is seen in

Phylum Ctenophora – The Comb Jellies

Imagine you are floating in the open ocean at night. Around you, tiny, almost invisible creatures drift, and when disturbed, they flash with brilliant blue-green light. If you could see them up close, you would notice they look like transparent, gelatinous bells or ribbons, and the most striking thing about them is the way they move — not by pulsing like a jellyfish, but by beating thousands of tiny, hair-like cilia arranged in eight neat rows that shimmer like rainbows. These are the ctenophores, or comb jellies.

The name itself tells you the key feature: cteno means "comb" in Greek, and phora means "bearing". So a ctenophore is a "comb-bearer", referring to those eight rows of ciliary combs.

The Intuition

Think of a ctenophore as a living, swimming kaleidoscope. It is one of the simplest animals that is truly multicellular, but it is already more complex than a sponge. It has a definite body plan: a mouth at one end, an anus at the other, and a simple gut in between. It is diploblastic — its body is made of only two germ layers (an outer epidermis and an inner gastrodermis), with a jelly-like layer called mesoglea sandwiched between them. It is radially symmetrical, meaning you can cut it through the centre in many planes and get mirror halves — though the symmetry is actually biradial, a subtle point you will meet later.

But the real magic is in the combs. Each of the eight rows is a band of fused cilia. When these cilia beat in coordinated waves, the animal glides forward. The beating also diffracts light, producing a rainbow effect that is not bioluminescence — it is just structural colour. The true bioluminescence comes from special cells that produce light when the animal is disturbed, a defence mechanism.

The Precise Statement

Key Features for Exams