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.
Nerve net
Nerve nets are found in species in the phyla Cnidaria (e.g. scyphozoa, box jellyfish, and sea anemones), Ctenophora, and Echinodermata. Cnidaria and Ctenophora both exhibit radial symmetry and are collectively known as coelenterates. Coelenterates diverged 570 million years ago, prior to the Cambrian explosion, and they are the first two phyla to possess nervous systems which differentiate during development and communicate by synaptic conduction. Most research on the evolution of nervous tissue concerning nerve nets has been conducted using cnidarians. The nervous systems of coelenterates allow for sensation, contraction, locomotion, and hunting/feeding behaviors. Coelenterates and bilaterians share common neurophysiological mechanisms; as such, coelenterates provide a model system for tracing the origins of neurogenesis. This is due to the first appearance of neurogenesis having occurred in eumetazoa, which was a common ancestor of coelenterates and bilaterians. A second wave of neurogenesis occurred after the divergence of coelenterata in the common ancestor of bilateria. Although animals with nerve nets lack a true brain, they have the ability to display complex movements and behaviors. The presence of a nerve net allows an organism belonging to the aforementioned phyla of Cnidaria, Ctenophora, and Echinodermata to have increased fitness as a result of being able to respond to their environment.