<P> The brain generally forms a ring round the pharynx (throat), consisting of a pair of ganglia (local control centers) above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it . The brains of polychaetes are generally in the prostomium, while those of clitellates are in the peristomium or sometimes the first segment behind the peristomium . In some very mobile and active polychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections . The rest of the central nervous system is generally "ladder - like", consisting of a pair of nerve cords that run through the bottom part of the body and have in each segment paired ganglia linked by a transverse connection . From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body . However, in most polychaetes the two main nerve cords are fused, and in the tube - dwelling genus Owenia the single nerve chord has no ganglia and is located in the epidermis . </P> <P> As in arthropods, each muscle fiber (cell) is controlled by more than one neuron, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons . Vertebrates have a different system, in which one neuron controls a group of muscle fibers . Most annelids' longitudinal nerve trunks include giant axons (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast . This enables these worms to withdraw rapidly from danger by shortening their bodies . Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement . </P> <P> The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body . Nuchal ("on the neck") organs are paired, ciliated structures found only in polychaetes, and are thought to be chemosensors . Some polychaetes also have various combinations of ocelli ("little eyes") that detect the direction from which light is coming and camera eyes or compound eyes that can probably form images . The compound eyes probably evolved independently of arthropods' eyes . Some tube - worms use ocelli widely spread over their bodies to detect the shadows of fish, so that they can quickly withdraw into their tubes . Some burrowing and tube - dwelling polychaetes have statocysts (tilt and balance sensors) that tell them which way is down . A few polychaete genera have on the undersides of their heads palps that are used both in feeding and as "feelers", and some of these also have antennae that are structurally similar but probably are used mainly as "feelers". </P> <P> Most annelids have a pair of coelomata (body cavities) in each segment, separated from other segments by septa and from each other by vertical mesenteries . Each septum forms a sandwich with connective tissue in the middle and mesothelium (membrane that serves as a lining) from the preceding and following segments on either side . Each mesentery is similar except that the mesothelium is the lining of each of the pair of coelomata, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it . The mesothelium is made of modified epitheliomuscular cells; in other words, their bodies form part of the epithelium but their bases extend to form muscle fibers in the body wall . The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut . Parts of the mesothelium, especially on the outside of the gut, may also form chloragogen cells that perform similar functions to the livers of vertebrates: producing and storing glycogen and fat; producing the oxygen - carrier hemoglobin; breaking down proteins; and turning nitrogenous waste products into ammonia and urea to be excreted . </P>

Most species in the phylum annelida live in what type of habitat