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Why do cephalochordates have no heart?

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A translucent X-ray fish (Pristella maxillaris) swims in an aquarium, showing its visible backbone and internal structures through its body. — Chordate X-ray fish (*Pristella maxillaris*), is an example of a chordate with a visible backbone.

Cephalochordates, such as the amphioxus, do not possess a single, centralized heart. Instead, they utilize a closed circulatory system where blood is propelled through the body by the rhythmic, independent contractions of various blood vessels.12

Rather than relying on one main pump, circulation is maintained by several key structures:
* Ventral Aorta: This primary vessel, located beneath the endostyle, forces blood forward through the body.1
* Branchial Hearts: These small, accessory structures are located at the gill arches and assist in pumping blood upward through the gills for gas exchange.2
* Other Vessels: Contractions also occur independently in the sinus venosus, the subintestinal vein, and other vascular channels to ensure continuous blood flow.2

This arrangement allows cephalochordates to circulate blood effectively without the need for a complex, centralized heart organ, relying instead on the coordinated action of these contractile vessels.12