The annelids achieved superior locomotion with a compartmentalized coelom, which allowed local control of the hydrostatic skeleton. The phylum Echinodermata have utilized this same principle, but to a more refined degree. Echinoderms posses a series of specialized hydrostatic structures known collectively as the water vascular system (WVS). The WVS is derived from the coelom and serves to carry sea water, the echinoderms' hydrostatic fluid, from the outside to the terminal WVS structure, the tube foot. We'll use the sea star as an example to illustrate the function of the WVS.

Like the cnidarians, the body plan of the echinoderms shows a pattern of radial symmetry. The basic and more or less equal units are arranged in a circle around a central disc. Many sea stars and other echinoderms have five of these similar units, giving them pentamerous (penta:five; mer:part) radial symmetry. Obtain a live sea star if they are available and note the five arms or rays extending from the central disc.

(A) External anatomy

The mouth of the sea star is located on its oral side, which faces downward. The opposite side is called the aboral surface and often has a highly textured, bumpy surface, sometimes with spines. These bumps and spines are part of the endoskeleton, which is covered by a layer of epidermis. The endoskeleton consists of individual ossicles which fit together closely in some species and more loosely in others. The tightness or looseness of fit determines the degree to which an echinoderm's body is stiff and tough, or soft and flexible. How does your specimen feel?

Turn your specimen so that oral surface, with the centrally located mouth, is up. You'll notice furrows running from the mouth along each ray. These are called the ambulacral grooves and are bordered by one or more rows of tube feet. The spiny endoskeleton is also visible on the sea star's oral surface.

(B) The Water-Vascular System

We are now ready to look at the WVS. The WVS runs from the aboral madreporite, where sea water is pumped into WVS by ciliary action, through the stone canal to the ring canal. From the ring canal the water goes through five radial canals, and finally to individual tube feet.

Each tube foot consists of a hollow, muscular structure attached to a balloon-like fluid reservoir called an ampulla. The elastic surface of the ampulla is covered with a mesh of muscle fibers and expands as water is pumped into it from the radial canal. Water is prevented from flowing back into the radial canal by a one-way valve. When the ampullar muscle fibers contract, the ampulla is deflated and water is forced into the tube foot. This stretches the tube foot's muscles and extends the tube foot beyond the ambulacral grove. Contraction of the tube foot muscles forces water back into the ampulla and stretches the ampullar muscles. Many sea stars have muscle fibers attached to the bottom of the foot. When the bottom of the tube foot is pressed against a solid surface, contraction of these muscle fibers creates a vacuum, allowing the foot to operate as a suction cup.

The WVS, involving hundreds of these tube feet, is the basis for the locomotor ability of the sea star. Each tube foot is under fine nervous control; not only can it be extended, but it can also be moved through a 360 degree arc by local contraction and relaxation of the tube foot musculature, playing against the hydraulic pressure from the ampulla. The sea star is capable of mostly quite slow, but very precise and well-coordinated movements on nearly any solid horizontal or vertical substratum. The system would not be effective, however, without some rigid structural support for the muscles. This is provided by the interconnecting ossicles of the endoskeleton.

A good way to view the action and coordination of the sea star's tube feet is to place it on its aboral surface in a dish of sea water. You'll see the flexibility of the sea star's endoskeleton as it bends one or more rays underneath itself, and the strength of the suckered tube feet as it pulls itself over to its oral-surface down position. Sea stars use their tube feet for prey capture and manipulation as well as locomotion. Other species of echinoderms employ their tube feet for a number of functions, but the basic hydraulic operation of the individual tube foot is the same.