Body Structure & Physiology







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Web Design By:

Kaili Jackson

University of Alaska Southeast Juneau, AK



The Water Vascular System A generalized view of the cucumber water vascular system. Borrowed from the Animal Diversity Website.

            The water vascular system is a unique and defining characteristic of echinoderms. It is a vital part of the system and exhibits control over the hundreds to thousands of tube feet that live in the ambulacral grooves. Tube feet (podia) participate in locomotion, gas exchange, feeding, attachment, and sensory reception. The water vascular system controls the tube feet hydraulically through a complex of fluid filled canals and reservoirs.
            Unlike other echinoderms, the madreporite of the holothuroideans is located internally below the pharynx and opens to the coelom instead of the outside world. The madreporite gives rise to a short stone canal. The stone canal is a vertical calcareous tube that connects the madreporite to the ring canal, which encircles the gut, just below the mouth.

The stone canal gives rise to polian vesicles that extend into the body cavity from the oral tentacles, which are modified podia. The oral tentacles are hydraulically controlled by the water pressure in the polian vesicles.  By contracting the polian vesicles, water is forced into the tentacles and they extend.

The ring canal extends into five radial canals that run down the ambulacral grooves of the cucumber, giving rise to rows of ampulla. Each external podia is isolated and operated by hydraulic pressure regulation and muscle action of the individual ampulla. When the ampulla is filled with fluid and the radial canal is closed, the ampulla will be able to contract and force fluid into the podia.  The fluid pressure forces the “sucker” on the bottom of the podia onto the substrate where the epidermis will secrete an adhesive substance. This holds the podia in place while the longitudinal muscles contract, pulling the cucumber forward and forcing the fluid back into the now relaxed ampulla. Meanwhile, a vacuum is created under the podia by muscle lifting the sucker disc on the bottom of the foot.

Release is mediated by the relaxation of podial muscles and the contraction of the ampulla; which forces fluid into the lumen, destroying the vacuum. The released podia will move to its next location and is capable of bending due to differential contraction of the longitudinal muscles.

Most suspension feeders move very little; usually they will find a rock or crevasse to attach to, more or less permanently.  In those scenarios, the podia are used more for attachment than they are for locomotion.

The dendritic tentacles and the tube feet of cucumaria vegae, both types of appendages are controlled by the water vascular system. Photo taken by Kaili Jackson.