Explain: $Ctenophora$.

(N/A) $Ctenophora$,commonly known as sea walnuts or comb jellies,are exclusively marine organisms.
They are diploblastic,radially symmetrical,and have tissue-level organization.
The body bears $8$ external rows of ciliated comb plates,which help in locomotion.
Digestion is both extracellular and intracellular.
Bioluminescence (the property of a living organism to emit light) is well-marked in $Ctenophora$.
Sexes are not separate (hermaphrodite).
Reproduction takes place only by sexual means.
Fertilization is external with indirect development.
Examples include $Pleurobrachia$ and $Ctenoplana$.