Box jellyfish (Cubomedusae), while particularly famous for their potent toxins, are also well known for their complex visual system. Sensory structures called rhopalia group no less than 24 eyes into sets of six, which include four morphological types. The upper and lower lens eyes are complex and resemble those of higher organisms. The pit eyes, on the other hand, are made up of a single cell type and are likely to only function in sensing of light levels, and the slit eyes, made up of four cell types, are possibly involved in basic spatial resolution.
The role of vision in box jellyfish has been previously established to be in phototaxis (movement in response to light stimulus), obstacle avoidance and control of swim rate. Now research is beginning to reveal the more complex uses of this extensive visual system. Studies of Tripedalia cystophora, a species of box jellyfish living in Caribbean mangrove swamps (pictured), have found that the upper lens eye always points directly upwards, regardless of bodily orientation - even if completely upside down - and is used to identify terrestrial structures, i.e. the mangrove canopy, and use them as navigational cues.
Pretty clever for a creature without a brain, eh?
Ref: Garm, Andersson & Nilsson (2008) Unique structure and optics of the lesser eyes of the box jellyfish Tripedalia cystophora. Vision Research 48(8) 1061-1073 [link]
Garm, Oskarsson & Nilsson (2011) Box Jellyfish Use Terrestrial Visual Cues for Navigation. Current Biology 21 798-803 [link]