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.
Sea turtle
Sea turtles' limbs and brains have evolved to adapt to their diets. One of the main things sea turtles consume is jellyfish and the use of their limbs to hold, swipe, and forage their food has helped them eat more efficiently. Originally sea turtles' limbs have evolved for locomotion but now they have evolved to aid them in the ability to get food. In addition to the evolution of limbs, sea turtles' brains have also evolved. The frontal cortex of a sea turtle's brain was not very developed. This underdevelopment caused the sea turtles to not be able to learn from their surroundings.