138 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”Further centralization of the nervous system led to the formation
of ganglia, which are aggregates of neurons numbering in hundreds to
thousands, as seen in worms (Fig. 7.7). These animals have a bilateral
body plan and a longitudinal axis with well-defined front and rear ends.
A ganglion is not a simple relay station of sensory and motor neurons.
Rather, it serves as an integrative center containing numerous associa-
tive neurons. Distributed along the longitudinal axis of the body, ganglia
perform semi-autonomous, regional functions, yet are part of a hier-
archy, subordinating to a head ganglion at the front end where major
Fig. 7.7. Nervous system of a flatworm, showing a head ganglion (with two lobes) in the
front part (top) and multiple smaller ganglia along the body. [See Note 33: Hyman LH.]