How does the cephalochordate atrium differ from the tunicate atrium?

In the context of the subphylum Cephalochordata and the subphylum Tunicata, the atrium is considered to have evolved independently in both groups, meaning there is little evidence to suggest they form a single lineage.1

While both cephalochordates and tunicates are filter feeders that utilize an atrium to process water currents, their structural development differs:

• Cephalochordates: The atrium serves as a chamber through which water, having passed through the pharynx and gill slits, exits the body.2
• Tunicates: The atrium typically forms during metamorphosis from a pair of pouches that grow inward and fuse into a single cavity, eventually opening near the mouth on the dorsal area of the body.3

It is important to note that the term "atrium" in these invertebrates refers to a cavity involved in water filtration, which is distinct from the atrium found in the vertebrate heart, which is a chamber for receiving and pumping blood.41