In amphioxus a big space is present, called Atrium it is lined by ectoderm. It encloses pharynx, and part of intestine. It opens out through atriopore at 36 myotome in front of caudal fin. The dorsal wall of the atrium is folded. ln the larva the atrium develops at the cost of longitudinal ridges called Metapleural folds on the ventral side of the body. They extend forward. The ridges of both the sides unite and atrium is developed.
In the adult the atrium is well developed. The pharynx opens into the atrium through gill slits. Water from pharynx enters into the atrium through gill slits. During this course respiration will take place.
This water will go out through atriopore.
The gametes are liberated into atrium. They are also, sent out through atriopore.