Nervous system
Nervous systems are found in most multicellular animals, but vary greatly in complexity. The only multicellular animals that have no nervous system at all are sponges, placozoans, and mesozoans, which have very simple body plans. The nervous systems of the radially symmetric organisms ctenophores (comb jellies) and cnidarians (which include anemones, hydras, corals and jellyfish) consist of a diffuse nerve net. All other animal species, with the exception of a few types of worm, have a nervous system containing a brain, a central cord (or two cords running in parallel), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to around 300 billion cells in African elephants.
Serpula
Like other annelids, these worms possess well-developed nervous systems. The nervous system consists of a central brain in the upper part of the head, which is relatively large compared with that of other annelids. Extending from the brain is a large ventral nerve cord running the length of the body. There are many supporting ganglia along the length of this cord (including pleural, pedal and cerebral ganglia), and a series of small nerves in each body segment. Signals transmitted through the pedal ganglia allow the worms to retract rapidly into their tube if threatened.