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Cnidaria consists of thousands of species, all living in marine aquatic environments. Some of the most known species of animals in this phylum are parasites and jellyfish. Since Cnidaria have no brain, their "nervous system" is quite complex and elaborate. Rather than having a formal central nervous system because of the absence of a brain, the Cnidaria has a decentralized network, called a nerve net. A nerve net allows an animal lacking a brain to respond to physical contact. A nerve net consists of connected neurons and are mainly associated with species of organisms with radical symmetry, such as jellyfish. With a nerve net in their bodies, Cnidaria can detect food and other chemicals in their way. Signals are generated in response types of stimulus, such as odors, which allow the Cnidaria to interact with their marine environments. Similar to Mollusca, Cnidaria also form ganglia, produced my immediate neurons, which serve as coordination centers. One of the most important aspects of the nerve net are motor neurons, which allow the muscles in the body to contract. This is essential to the movement of the organism, and allowing it to respond to their environment. Cnidaria also contain a cell called cnidocytes, which detect physical contact. Cnidocytes are informed from nerves whenever an odor is detected, or an attacker is detected. This is essential in regard to the feeding routines of the Cnidaria, and their ability to survive. All of this communication between the nerves are via chemical synapses, which are small gaps in the internal body of the Cnidaria, which chemicals flow through. Swimming colonies such as siphonophores and chondrophores allow Cnidaria to sense when their bodies tilt, and when their speeds accelerate by means of statocysts. Statocysts are small chambers within the Cnidaria's body, lined with microscopic hairs that detect the movement of mineral grains called statoliths. Once it is recognized that the body is tilting, the Cnidaria will adjust it's strength to the weakening side, therefore balancing the body. Certain species of Cnidaria have ocelli, which serve as eyes that detect sources of light.