Circulation, Respiration and Excretion of Asteroidea
Asteroids depend upon the circulation of coelomic fluid for the transport of gases and some nutrients. The blood vascular system termed as haemal system in echinoderms is not very well developed in asteroids. The channels lack a lining. Though, these channels are surrounded by extensions of coelom called perihaemal sinuses. There are four (4) coelomic circulatory systems in asteroids.
(1) The perivisceral coelom and the fluid that circulates in it that supplies the viscera
(2) The water vascular system which supplies the muscles of the tube feet that aids in locomotion
(3) Hyponeural sinus system which supplies the nervous system
(4) Genital coelom that supplies the genital organs.
Echinoderms live completely in sea water with which their body fluid is isotonic. The nitrogenous excretory wastes are removed through the thin areas of body surface, such as tube feet and papulae by diffusion. Besides, coelomocytes as well play a role in removing the metabolic wastes from the coelom. Gas exchange in asteroids is achieved by papulae (dermal branchiae), soft projections of coelomic cavity standing out by the spaces between ossicles. Tube feet are also principal respiratory organs.