These control contraction of circular or longitudinal muscle. Fig 11-3 p 409. Organisms as high in the evolutionary tree as molluscs have nerve cells that are both sensory and motor. They can effect a cell without connecting to a ganglion. Molluscs also have ganglia where coordination can take place.
Some molluscs show cephilization in their nerve gangli.
Arthropods have ganglia more complex and joined in the cephalid end and ganglia that are connected via nerve trunks. There is a set ganglia in each segment of the animal located down the ventral surface of the body. This nerve cord is solid. Examination of the nervous systems of different phyla indicate that more complex nervous systems are produced by adding on to existing nervous structures. The Vertebrate Nervous System Note Figure 11-6 p 412 for general organization of the vertebrate nervous system.