Practice question

Starfish respiration relies on papulae, also called dermal branchiae, numerous thin-walled outpocketings of coelom protruding between skeletal ossicles of aboral surface. Each papula consists of outer ciliated epidermis and inner coelomic lining with minimal barrier, highly vascularized for oxygen diffusion from seawater into coelomic fluid and hemal system. Papulae are retractable to avoid fouling and predation, supplemented by gas exchange across tube feet. Unlike tracheal tubes of insects, book lungs of arachnids or lamellate gills of fishes, papular respiration suits sessile benthic marine life with low metabolic demands.