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.
Evolution of nervous systems
Iodine and T4 stimulate the spectacular apoptosis (programmed cell death) of the cells of the larval gills, tail and fins, and also stimulate the evolution of the nervous system transforming the aquatic, vegetarian tadpole into the terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting. Contrary to amphibian metamorphosis, thyroidectomy and hypothyroidism in mammals may be considered a sort of phylogenetic, metabolic and neurologic regression to a former stage of reptilian life. Indeed, many disorders that seem to afflict hypothyroid humans have reptilian-like features, such as a general slowdown of nervous reflexes with lethargic cerebration, metabolism, digestion, heart rate, hypothermia and a dry, hairless, scaly, cold skin.
Behaviors such as the "tail-flip" escape reaction in crustacea such as crayfish and lobsters are fixed action patterns that may have evolved from earlier ancestral patterns.