<P> Neurons differ in invertebrates from mammalian cells . Invertebrates cells fire in response to similar stimuli as mammals, such as tissue trauma, high temperature, or changes in pH . The first invertebrate in which a neuron cell was identified was the medicinal leech, Hirudo medicinalis . </P> <P> Learning and memory using nociceptors in the sea hare, Aplysia has been described . Mollusk neurons are able to detect increasing pressures and tissue trauma . </P> <P> Neurons have been identified in a wide range of invertebrate species, including annelids, molluscs, nematodes and arthropods . </P> <P> One type of invertebrate respiriatory system is the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues . The distribution of spiracles can vary greatly among the many orders of insects, but in general each segment of the body can have only one pair of spiracles, each of which connects to an atrium and has a relatively large tracheal tube behind it . The tracheae are invaginations of the cuticular exoskeleton that branch (anastomose) throughout the body with diameters from only a few micrometres up to 0.8 mm . The smallest tubes, tracheoles, penetrate cells and serve as sites of diffusion for water, oxygen, and carbon dioxide . Gas may be conducted through the respiratory system by means of active ventilation or passive diffusion . Unlike vertebrates, insects do not generally carry oxygen in their haemolymph . </P>

The only group of invertebrates that can fly