Cubozoa are very similar to the "True Jellyfish" but it has a square bell in cross section(the top of the Cubozoa) and they have a velum-like structure called the velarium.
This structure restricts the size of the opening through which water is pushed out, for propulsion. Thus making these Cubozoas faster swimmers than the "True Jellyfish".
Cubozoas also have miniature eyes called ocelli. They have many of these eyes around the bell, around 20 in fact. These eyes are not complex and cannot detect images so these are most likely for light detection and obstacle awareness.
These animals also have very complex eyes with lenses, corneas and retinas. Their eyes are capable of producing very high quality images almost as good as humans eyes .(http://eol.org/pages/6681/overview). Each eye is positioned on each of the four corners of the square shaped bell increasing their visual awareness.
Some Cubozoas also can have very venomous stingers. Some Cubozoans have been nicknamed "sea wasp" and "marine stinger" because of these infamous stingers.