This slide shows a stained cross section through the pharyngeal region of the lancelet Amphioxus.  Observe the dorsal fin supported by a fin ray and metapleural folds on the ventral surface that also help to stabilize the lancelet during week swimming movements.  Note the conspicuous myomeres (muscle blocks) and gonads as well as the large notochord and dorsal nerve cord above it.  Lancelets are filter-feeders, and water brought in through the mouth enters a large, perforated pharynx where particulate matter is removed.  From the pharynx, water passes through gill slits supported by gill bars into a surrounding chamber called the atrium to exit through a posterior atriopore (not seen on the cross section).