Instruct: Given a question, retrieve passages that answer the question.
Query: Do jelly fish have nervous systems?
Most jellyfish do not have specialized systems for osmoregulation, respiration and circulation, and do not have a central nervous system. Nematocysts, which deliver the sting, are located mostly on the tentacles; true jellyfish also have them around the mouth and stomach.[30] Jellyfish do not need a respiratory system because sufficient oxygen diffuses through the epidermis. They have limited control over their movement, but can navigate with the pulsations of the bell-like body; some species are active swimmers most of the time, while others largely drift.[31] The rhopalia contain rudimentary sense organs which are able to detect light, water-borne vibrations, odour and orientation.[22] A loose network of nerves called a "nerve net" is located in the epidermis.[32][33] Although traditionally thought not to have a central nervous system, nerve net concentration and ganglion-like structures could be considered to constitute one in most species.[34] A jellyfish detects stimuli, and transmits impulses both throughout the nerve net and around a circular nerve ring, to other nerve cells. The rhopalial ganglia contain pacemaker neurones which control swimming rate and direction.[22]
The neologism pescetarians covers those who eat fish and other seafood, but not mammals and birds. Pescatarians may consume fish based solely upon the idea that the fish are not factory farmed as land animals are (i.e., their problem is with the capitalist-industrial production of meat, not with the consumption of animal foods themselves). Some eat fish with the justification that fish have less sophisticated nervous systems than land-dwelling animals. Others may choose to consume only wild fish based upon the lack of confinement, while choosing to not consume fish that have been farmed.
Recent crystal structures of Rev, the Rev-ARM/Stem IIB structure and the information on Rev-RRE stoichiometry have led to the proposal of a jelly-fish model for a functionally active complex. In this model, The RRE provides a structural scaffold to assemble a Rev hexamer, and this assembly forms the head of the jelly-fish. The NESs from the 6 Rev monomers form the jelly-fish "tentacles" that could interact with the host Crm1-RanGTP proteins. This entire "jellyfish" would then be exported to the cytoplasm (see Figure).
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, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete ganglia.
Vertebrates are the only chordate group to exhibit a proper brain. A slight swelling of the anterior end of the dorsal nerve cord is found in the lancelet, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation. The central nervous system is based on a hollow nerve tube running along the length of the animal, from which the peripheral nervous system branches out to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the central canal of spinal cord into three primary brain vesicles: The prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups. Two laterally placed eyes form around outgrows from the midbrain, except in hagfish, though this may be a secondary loss. The forebrain is well developed and subdivided in most tetrapods, while the midbrain dominate in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in mammals. The resulting anatomy of the central nervous system, with a single, hollow ventral nerve cord topped by a series of (often paired) vesicles is unique to vertebrates.
Contrary to popular misconception, opioids are not depressants in the classical sense. They do produce central nervous system depression, however, they also excite certain areas of the central nervous system. To remain true to the term 'depressant' – opioids cannot be classified as such. For opioid agonists and opium derivatives, these are classified differently. Analgesic or narcotic correctly identifies these drugs. However, they do have depressant actions nonetheless.
Combining multiple depressants can be very dangerous because the central nervous system's depressive properties have been proposed to increase exponentially instead of linearly. This characteristic makes depressants a common choice for deliberate overdoses in the case of suicide. The use of alcohol or benzodiazepines along with the usual dose of heroin is often the cause of overdose deaths in opiate addicts.
Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of a synapse). However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.
Uniquely, by the time "M. mymaripenne" reaches adulthood, 95% of its nervous cells have lost their nuclei. Only 339–372 nuclei are found throughout the central nervous system, of which 179–253 are found in the brain. The nervous system of the pupae of "M. mymaripenne" makes up 19% of its body mass, 11% of which is the brain. Unlike in adults, cells in the pupae have nuclei. Only in the final stage of development do these undergo lysis, which greatly reduces the volume of the nervous system. While the brains of pupae are 93,600 μm, those of the adults are only 52,200 μm. Accompanying this shrinkage of brain volume is a shrinking of the occipital area of the head, with the cuticle folding into helical spirals.
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.
Principal regions of the vertebrate brain:
Peripheral nervous system (PNS) – nervous system structures that do not lie within the CNS.
The autonomic nervous system has three branches: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. Some textbooks do not include the enteric nervous system as part of this system. The sympathetic nervous system is often considered the "fight or flight" system, while the parasympathetic nervous system is often considered the "rest and digest" or "feed and breed" system. In many cases, both of these systems have "opposite" actions where one system activates a physiological response and the other inhibits it. An older simplification of the sympathetic and parasympathetic nervous systems as "excitatory" and "inhibitory" was overturned due to the many exceptions found. A more modern characterization is that the sympathetic nervous system is a "quick response mobilizing system" and the parasympathetic is a "more slowly activated dampening system", but even this has exceptions, such as in sexual arousal and orgasm, wherein both play a role.
"M. cerebralis" infections have been reported from a wide range of salmonid species: eight species of "Atlantic" salmonids, "Salmo"; four species of "Pacific" salmonids, "Oncorhynchus"; four species of char, "Salvelinus"; the grayling, "Thymallus thymallus"; and the huchen, "Hucho hucho". "M. cerebralis" causes damage to its fish hosts through attachment of triactinomyxon spores and the migrations of various stages through tissues and along nerves, as well as by digesting cartilage. The fish's tail may darken, but aside from lesions on cartilage, internal organs generally appear healthy. Other symptoms include skeletal deformities and "whirling" behavior (tail-chasing) in young fish, which was thought to have been caused by a loss of equilibrium, but is actually caused by damage to the spinal cord and lower brain stem. Experiments have shown that fish can kill "Myxobolus" in their skin (possibly using antibodies), but that the fish do not attack the parasites once they have migrated to the central nervous system. This response varies from species to species.
Pavlov's background as a physiologist greatly influenced his psychological research. His ideas for the how the nervous system works came from his research on the brain, specifically the cortex, and conditioned and unconditioned reflexes, and spurred additional research into the nervous activity that underlies such reflexes. Pavlov introduced the ideas of strength of excitation and inhibition, as well as mobility, irradiation, and generalization in the central nervous system. 
Strength of excitation was considered to be the most important of the nervous system properties by Pavlov because we are often confronted by stimuli in the environment that grab our attention immediately. Excitation of the cells is simply their ability to do work, and this can either be very short-lived or extended and have varying intensities. This excitation can lead to the gain of conditioned responses, or the generalization of a conditioned response. Inhibition, on the other hand, is when a response that is already encoded is overcome or contained. Mobility is the ability of the nervous system to give one impulse priority over another. This can be done by having excitation before inhibition, or vice versa.
Pavlov looked at the balance between excitation and inhibition and determined that there were different types of nervous systems. These observations were made about dogs, but Pavlov believed that they could be generalized to humans as well. The different types of nervous systems were categorized by Pavlov into strong and weak nervous systems with different sub-types under the strong type. These different types of systems were categorized based on the strength of excitation or inhibition in the system of the observed individual. Those individuals with weak nervous systems were thought by Pavlov "to have limited adaptive abilities," while strong nervous systems were more adaptable.
Although Pavlov's typology originated as a purely conceptual idea, modern technology has helped research in the area to provide evidence for the processes of higher nervous activity that Pavlov hypothesized.
Sponges have no cells connected to each other by synaptic junctions, that is, no neurons, and therefore no nervous system. They do, however, have homologs of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a postsynaptic density (the signal-receiving part of a synapse). However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.
Emotional pain activates the same regions of the brain as physical pain, so emotional stress can be a significantly intolerable state for some people. Some of this is environmental and some of this is due to physiological differences in responding. The autonomic nervous system is composed of two components: the sympathetic nervous system controls arousal and physical activation (e.g., the fight-or-flight response) and the parasympathetic nervous system controls physical processes that are automatic (e.g., saliva production). The sympathetic nervous system innervates (e.g., is physically connected to and regulates) many parts of the body involved in stress responses. Studies of adolescents have shown that adolescents who self-injure have greater physiological reactivity (e.g., skin conductance) to stress than adolescents who do not self-injure. This stress response persists over time, staying constant or even increasing in self-injuring adolescents, but gradually decreases in adolescents who do not self-injure.
Hair cells in fish are used to detect water movements around their bodies. These hair cells are embedded in a jelly-like protrusion called cupula. The hair cells therefore can not be seen and do not appear on the surface of skin.
These fish have muscular bodies, ossified bones, scales, well developed gills and central nervous systems, and large hearts and kidneys. Mesopelagic plankton feeders have small mouths with fine gill rakers, while the piscivores have larger mouths and coarser gill rakers. The vertically migratory fish have swimbladders.
Nervous systems are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.
Sponges are animals of the phylum Porifera (Modern Latin for "bearing pores" ). They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells. They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. Sponges do not have nervous, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes.
Midshipman fish have two forms of males: the nest-building Type I and "sneaker male" or "satellite male" Type II. Type I males attract females to their nests with their humming, coax them to lay eggs, and guard them. In contrast, Type II males do not build nests or attract females on their own. Instead, they sneak up to the Type I's nests and deposit their eggs. These behavioral differences can be seen in the differences in the structure and function of the nervous system.
Decomposition of jelly-falls is largely aided by these kinds of scavengers. In general, echinoderms, such as sea stars, have emerged as the primary consumer of jelly-falls, followed by crustaceans and fish. However, which scavengers find their way to jelly-falls is highly reliant on each ecosystem. For example, in an experiment in the Norwegian deep sea, hagfish were the first scavengers to find the traps of decaying jellies, followed by squat lobsters, and finally decapod shrimp. Photographs taken off the coast of Norway on natural jelly-falls also revealed caridean shrimp feeding on jelly carcasses.
Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges' bodies consist of a non-living jelly-like mass (mesoglea) sandwiched between two main layers of cells. Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as "basal lamina"). Sponges have no nervous systems, their middle jelly-like layers have large and varied populations of cells, and some types of cells in their outer layers may move into the middle layer and change their functions.
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.
These fish have muscular bodies, ossified bones, scales, well developed gills and central nervous systems, and large hearts and kidneys. Mesopelagic plankton feeders have small mouths with fine gill rakers, while the piscivores have larger mouths and coarser gill rakers. The vertically migratory fish have swimbladders.
Neuroanatomy is the study of the structure and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems. Their neuroanatomy is therefore better understood. In vertebrates, the nervous system is segregated into the internal structure of the brain and spinal cord (together called the central nervous system, or CNS) and the routes of the nerves that connect to the rest of the body (known as the peripheral nervous system, or PNS). The delineation of distinct structures and regions of the nervous system has been critical in investigating how it works. For example, much of what neuroscientists have learned comes from observing how damage or "lesions" to specific brain areas affects behavior or other neural functions.