Nervous system
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.
Adult neurogenesis
Zebrafish have long been a classical developmental model due to their transparency during organogenesis and have been utilized heavily in early development neurogenesis.). The zebrafish displays a strong neurogenerative capacity capable of regenerating a variety of tissues and complete neuronal diversity (with the exception of astrocytes, as they have yet to be identified within the zebrafish brain) with continued neurogenesis through the life span. In recent decades the model has solidified its role in adult regeneration and neurogenesis following damage. The zebrafish, like the axolotl, has played a key role as a bridge organism between invertebrates and mammals. The zebrafish is a rapidly developing organism that is relatively inexpensive to maintain, while providing the field ease of genetic manipulation and a complex nervous system.