Atolla jellyfish
The body of "Atolla wyvillei" is a bell shape and it has several moderately long tentacles along the bell. The bell is around 20–174 mm in diameter. It also has a single long tentacle called a hypertrophied tentacle. This hypertrophied tentacle has several purposes, including aid in predation and aid in reproduction. These jellyfish do not have a digestive system, a respiratory system, a circulatory system or a central nervous system. Not having a central nervous system means that it lacks a brain.
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.