How does blood circulate in a cephalochordate without a heart?

Cephalochordates, such as the amphioxus (Branchiostoma lanceolatum), lack a centralized heart. Instead, they maintain a closed circulatory system through independent, rhythmic contractions occurring in various vessels and structures throughout the body.12

Blood circulation is facilitated by the following mechanisms:

• Contractile Vessels: The primary movement of blood is driven by the ventral aorta, which is located beneath the endostyle, and other contractile blood vessels.12
• Accessory Hearts: Small, accessory branchial hearts are located along the gill arches to pump blood upward through the gills for oxygenation.2
• Regional Contractions: Transport is further supported by contractions that arise independently in the sinus venosus, the subintestinal vein, and other vessels.2

In this system, blood flows forward through the ventral vessels and backward through the dorsal vessels. The blood is nonpigmented, contains no corpuscles, and transports oxygen via simple solution.21