[1][2] The common feature of this class is a pair of long, feathery, contractile tentacles, which can be retracted into specialised ciliated sheaths.

The tentacles have colloblasts, which are sticky-tipped cells that trap small prey.

The group includes the small, oval sea gooseberries found on both Atlantic and Pacific coasts.

The more flattened species of the genus Mnemiopsis, about 4 inches (10 cm) long, are common on the upper Atlantic coast; it has a large mouth and mainly feeds on larval molluscs and copepods.

Venus girdle (genus Cestum) is a flattened, ribbon-like form reaching over 3 feet (91 cm) in length, and found in tropical waters.