Nervous system
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, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete ganglia.
Irukandji syndrome
Most stings occur during the summer wet season in October–May in North Queensland, with different seasonal patterns elsewhere. Because jellyfish are very small, the venom is only injected through the tips of the nematocysts (the cnidocysts) rather than the entire lengths; as a result the sting may barely be noticed at first. It has been described as feeling like little more than a mosquito bite. The symptoms, however, gradually become apparent and then more and more intense in the subsequent 5 to 120 minutes (30 minutes on average). Irukandji syndrome includes an array of systemic symptoms, including severe headache, backache, muscle pains, chest and abdominal pain, nausea and vomiting, sweating, anxiety, hypertension, tachycardia and pulmonary edema. Symptoms generally improve in four to 30 hours, but may take up to two weeks to resolve completely.