Tube feet

Anatomical structures in echinoderms used for locomotion and feeding

Tube feet are small, flexible, hollow appendages found in echinoderms, such as starfish, sea urchins, and sea cucumbers. These structures are part of the water vascular system, a hydraulic system unique to echinoderms, and play a crucial role in locomotion, feeding, and respiration.

Structure

Tube feet are typically elongated, cylindrical structures that extend from the ambulacral grooves on the oral surface of echinoderms. Each tube foot consists of an internal ampulla and an external podium. The ampulla is a bulbous structure located inside the body cavity, while the podium extends outside the body and is often equipped with a suction cup at its distal end.

File:Starfish tube feet.jpg

Close-up of starfish tube feet showing the suction cups.

The walls of the tube feet are composed of connective tissue and muscle fibers, allowing them to contract and extend. The inner surface is lined with cilia, which help in the movement of water within the tube foot.

Function

Tube feet serve multiple functions in echinoderms:

Locomotion

Tube feet are primarily used for locomotion. By coordinating the extension and contraction of the tube feet, echinoderms can move across the substrate. The suction cups at the ends of the tube feet adhere to surfaces, allowing the organism to pull itself forward.

Feeding

In addition to locomotion, tube feet are used in feeding. For example, starfish use their tube feet to pry open the shells of bivalves such as clams and mussels. Once the shell is opened, the starfish can evert its stomach to digest the prey externally.

Respiration

Tube feet also play a role in respiration. The thin walls of the tube feet allow for gas exchange, facilitating the diffusion of oxygen into the body and the removal of carbon dioxide.

Water Vascular System

The water vascular system is a network of fluid-filled canals that facilitate the movement of tube feet. Water enters the system through the madreporite, a sieve-like structure on the aboral surface. From there, water is channeled into the stone canal, which leads to the ring canal surrounding the mouth. Radial canals extend from the ring canal into each arm, supplying water to the tube feet.

The ampullae of the tube feet contract to force water into the podium, extending it. When the ampullae relax, water is drawn back, retracting the tube foot.

Evolutionary Significance

The development of tube feet is a significant evolutionary adaptation that has allowed echinoderms to exploit a variety of ecological niches. Their ability to adhere to surfaces and manipulate objects has enabled them to become effective predators and scavengers in marine environments.

Related pages

• Echinoderm
• Water vascular system
• Starfish
• Sea urchin
• Sea cucumber