Chondrichthyes
Chondrichthyes nervous system is composed of a small brain, 8-10 pairs of cranial nerves, and a spinal chord with spinal nerves. They have several sensory organs which provide information to be processed. Ampullae of Lorenzini are a network of small jelly filled pores called electroreceptors which help the fish sense electric fields in water. This aids in finding prey, navigation, and sensing temperature. The Lateral line system has modified epithelial cells located externally which sense motion, vibration, and pressure in the water around them. Most subspecies have large well-developed eyes. Also, they have very powerful nostrils and olfactory organs. Their inner ears consist of 3 large semicircular canals which aid in balance and orientation. Their sound detecting apparatus has limited range and is typically more powerful at lower frequencies. Some subspecies have electric organs which can be used for defense and predation. They have relatively simple brains with the forebrain not greatly enlarged. The structure and formation of myelin in their nervous systems are nearly identical to that of tetrapods, which has led evolutionary biologists to believe that Chondrichthyes were a cornerstone group in the evolutionary timeline of myelin development.
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.