Answer to Question #54682 in Human Anatomy and Physiology for chandan
Describe the gills as respiratory organs in non-chordates. Explain the organization of nervous system in non-chordates.
Most non-chordates use skin, and tracheal systems for respiration. Gills are common for the aquatic annelides, molluscs and arthropoda. Most molluscs have only one pair of gills, or even only one gill. Generally, the gills are rather like feathers in shape, although some species have gills with filaments on only one side. They divide the mantle cavity so water enters near the bottom and exits near the top. Their filaments have three kinds of cilia, one of which drives the water current through the mantle cavity, while the other two help to keep the gills clean. If the osphradia detect noxious chemicals or possibly sediment entering the mantle cavity, the gills' cilia may stop beating until the unwelcome intrusions have ceased. Each gill has an incoming blood vessel connected to the hemocoel and an outgoing one to the heart. Some aquatic annelids have thin-walled, feathery gills through which gases are exchanged between the blood and the environment. By contrast, the polychaetes have several different kinds of respiratory organs. The sedentary polychaetes may use their tentacles as respiratory surfaces or may have specialized gill structures protruding from their tubes. In insects, gills are usually outgrowths of the tracheal system. they are covered by a thin layer of cuticle that is permeable to both ozygen and carbon dioxide.
Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-cell and colonial eukaryotes. Simple nerve nets seen in animals like cnidaria evolved first, followed by nerve cords in bilaterial animals - ventral nerve cords in invertebrates. Overall, the organisation of nervose system progressed from the individual neurons through nerve nets (jellyfish) to nerve cords (worms), nerve chains (insects).