The water vascular or ambulacral groove system is the most distinctive feature of echinoderms. It is a modified part of the coelom consisting of a network of canals which are filled with a watery fluid. Different types of canals combine together to form the system. It consists of a madreporite or sieve plate, stone canal, ring canal, radial canals and tube feet. The madreporite permits the entry of water. The system acts as the circulatory system, maintaining the continuous flow of water in and out of the body.
The flow of water is: Seawater → Madreporite → Stone canal (lined by ciliated epithelial) → Ring canal → Ambulacral system
The water vascular system is involved in three primary functions: locomotion, capture and transport food and respiration. Locomotion: The most peculiar and interesting role of the water vascular system is in locomotion. Tube feet are small tube-like projections on the present on the oral side of the echinoderm, arranged in grooves along the arm. The body moves with a mechanism called the stepping action of tube feet, which alternatively adhere to and release from the substratum on which the animals move. This happens with the entry and exit of water. Capture and transport of food: Echinoderms capture the prey and hold it in position with the help of its arms and tube feet. The food is taken in and then transported into the body with the water entering through the sieve plate. Respiration: The tube feet also serve as the equivalent of gills during respiration. The thin walls of the tube feet help in the respiratory exchange of gases. They absorb oxygen from water and pour carbon dioxide in it.
