Ctenidia is found in the order

Ctenidia is found in the order Siphonoptera.
The order Siphonaptera, includes 2,500 species of small flightless insects that live as external parasites of mammals and birds.
A ctenidium is shaped like a comb or a feather, with a central part from which many filaments or plate-like structures protrude, lined up in a row. It hangs into the mantle cavity and increases the area available for gas exchange.