Evolution of nervous systems
Jellyfish, comb jellies, and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons that pick up chemical, tactile, and visual signals, motor neurons that can activate contractions of the body wall, and intermediate neurons that detect patterns of activity in the sensory neurons and send signals to groups of motor neurons as a result. In some cases groups of intermediate neurons are clustered into discrete ganglia.
Jellynose fish
Together with their relatives, the Stomiiformes, the jellynoses are often placed in the teleost superorder Stenopterygii. Whether it is indeed justified to accept such a small group is doubtful; it may well be that the closest living relatives of the "Stenopterygii" are found among the superorder Protacanthopterygii, and that the former would need to be merged in the latter. In some classifications, the "Stenopterygii" are kept separate but included with the Protacanthopterygii and the monotypic superorder Cyclosquamata in an unranked clade called Euteleostei. That would probably require splitting two additional monotypic superorders out of the Protacanthopterygii, and is probably not ideal due to the profusion of very small taxa it would create. In fact, in some treatments, the jellynose fishes are even placed in yet another monotypic superorder, the Ateleopodomorpha.