Self And The Phenomenon Of Life: A Biologist Examines Life From Molecules To Humanity

The Animal Self: Neurobehavioral Correlates 137

“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity

Like the hydra, the free-swimming umbrella-like jellyfish has a
radial body plan, but the neurons show discrete synaptic transmission.
Instead of a diffusely distributed nerve net, jellyfish neurons tend to
group together. Some exhibit endogenous rhythmic activities. Some
nerve processes become longer than others and are arranged in parallel
bundles for rapid and unidirectional conduction. The nerve nets evolved
into nerve rings, which receive messages from well-defined sense organs
and discharge into swimming musculature (Fig. 7.6). There are two
types of sense organs: one is light sensitive which detects the direction of
sunlight, the other responds to gravity with respect to body orientation.
Refinement of the neuromuscular system enables a jellyfish to engage
in free swimming and procuring food by coordinated movements, con-
sisting of alternate contractions of the circular and longitudinal muscles,
expansion of the disc and elongation of the stalk, and swaying move-
ments of the body.

Fig. 7.6. Nerve ring of a jellyfish. (1) Upper nerve; (2) fibers crossing the upper and
lower nerves; (3) lower nerve; (4) connecting fibrils to sub-umbrella net. The dome of
the jellyfish umbrella is oriented toward the top. [See Note 32: Hyman LH. (1940); after
Hyde I. (1902).]