Cubomedusae, or box jellyfish, are renowned for their immense stinging

power, but another truly remarkable feature is their visual system. They

have four sensory structures called rhopalia, and each of the rhopalia contains

six eyes of four morphological types. These eyes support a range of

behaviors in the box jellyfish such as obstacle avoidance and navigation.

The need to process the visual information and turn it into the appropriate

behavior puts strong demands on the nervous system of box jellyfish,

which appears more elaborate than in other cnidarians. Here, the central

part of this nervous system is described. Each rhopalium holds a separate

part of the CNS with 1,000 nerve cells and a large amount of neuropil.

The rhopalial nervous system has several subsystems defined by the anatomy,

location, and immunocytochemistry of the cells. Most of the subsystems

connect to one or more of the eye types, and it is likely that the rhopalial

nervous system accounts for most of the visual processing. The

major part of the CNS is made up of a ring nerve encircling the bell

shaped body. The ring nerve holds around 10,000 cells and is directly

connected to all four rhopalial nervous systems. The main function of the

ring nerve is probably to integrate the information from the rhopalia and

control the motor nerve net. Both parts of the CNS contain subsystems of

giant neurons indicating the need for fast conduction. Uni- as well as

bidirectional synapses are found throughout the CNS.