Scyphozoa (true jellyfish) are much more interesting (in a neurobiological way) than previously described corals. One major difference is that Scyphozoa are pelagic animals, which means they are not fixed to the ground. They also have two diffuse nerve nets (subepidermal and subgastrodermal) that consist of bipolar and multipolar neurons – the impulse conduction has been measured at 0,15 m/s. Both nets coordinate the movements of an animal towards the food. Some scientists, however, differentiate one diffuse and one motor nerve net. The motor net is in charge of the activation of muscle contractions after receiving signals from the so-called pacemaker organs (which are in charge of the swimming rhythm). The diffuse net, in this case, is in charge of marginal tentacle contraction and it is also believed it communicates sensory information to jellyfish musculature. Neurons of the motor nerve net are connected by chemical synapses, while neurons of diffuse nerve net are connected by peptidergic synapses that were noted in Anthozoa as well.
Sycphozoa also have much more developed sensory organs than any of the animals previously mentioned. These sensory structures are called rhopalia and they are located on the edges of the jellyfish bell – there are usually four of them (or a number that’s a multiple of four). Rhopalia contain multiple sensory receptors – statocyst (balance receptor), ocelli (light sensitivity), a mechanoreceptor, a chemoreceptor, and aforementioned pacemaker neurons.
I would also like to note here that some authors (I’m referring here to the article “Do jellyfish have central nervous systems?” by R. A. Satterlie) believe this kind of nerve net explanation is rather simplified and that there exist some evidence suggesting that jellyfish have a centralized nervous system, mainly that rophalia are in fact rudimentary ganglia and could be regarded as integrative centers. However, any communications between rhopalia themselves exist only through the nerve nets.
Literature & more information:
Habdija et al: Protista-Protozoa, Metazoa-Invertebrata, Alfa, 2011, Zagreb
Do jellyfish have central nervous systems?
Jellyfish nervous systems