The system through which water and blood flow is called circulatory system. Water is the main means of transportation for underdeveloped animals. Animals of the group Periphera have a vascular system and animals of the group Echinodermata have a water circulatory system. The main means of circulation in advanced animals is blood or liquid connective tissue. There are two types of circulatory system.
(i) Closed circulatory system: The circulatory system in which blood always flows through the vessels is called closed circulatory system. The circulatory system of animals of class Annelida and Chordata is closed type.
(ii) Open circulatory system: The circulatory system in which blood flows through different parts of the body is called open circulatory system. The circulatory system of Arthropoda and Mollusca animals is open type.