Deuterostome Diversion

We now make the case for the large, three-dimensional hemocoels of sea urchins and starfish. In a three-dimensional volume, there is only about a thirty-four percent chance for molecules to traverse the cavity from one location to another without getting lost; so how can deuterostomes receive adequate energy if two-thirds of their nutrient molecules get lost in the infinities of their internal oceans of coelomic fluid? The answer must involve the idea that echinoderms, being exclusively marine, lead lives that are slow and cold enough for diffusion coupled with some minimal sloshing of fluid to supply their metabolism. Echinoderms move slowly and require less energy than faster moving forms. Beating cilia on the walls of coeloms may contribute a weak current.

For zoological reasons, the cavity of a starfish or sea urchin is a coelom and not a hemocoel, but coeloms work like hemocoels. However, all is not so simple in echinoderm larvae that possess gel-filled cavities (Ref: Gel-Filled Cavities).

To observe the movements and fates of particles in the coelom of a starfish such as Asterias, inject less than a milliliter of a carmine suspension in seawater into one of the rays of a living starfish through a hypodermic syringe attached to a twenty-gauge needle. See the red color rapidly traverse the ray to enter the body. You will see how the cilia on the walls of the coelom create an internal circulation.

After ten minutes, withdraw a few milliliters of coelomic fluid from the starfish and examine this fluid under a compound microscope. You will see that amoeboid phagocytic cells in the fluid are engulfing the particles of carmine. Along the margins of the radial canal (remember this is the hydrostatic system having to do with activating the tube feet) are nine areas, called Tiedemann's Bodies. These structures probably filter the coelomic fluid because they remove the carmine-filled cells.

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