136 Self and the Phenomenon of Life
b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”millimeters long with a base attached to any fixed structure in the water.
It has a single upward opening, surrounded by tentacles, that serve
as both a mouth and an anus. Upon contact with a prey, the tentacles
capture it with hook-like structures and bring it into the mouth for
digestion. The indigestible residue is subsequently extruded through
the same opening. When startled, the hydra retracts into a small ball, a
stereotype response irrespective of the direction of the stimulus. Hydra
has a nerve net made up of primitive nerve-like cells that conduct a slow
action potential (Fig. 7.5B). The cells are interconnected but these con-
nections do not show the full characteristics of a synapse, as there is no
one-way transmission, and the impulse appears to diminish in strength
along the course. These conducting cells are diffuse and equally distrib-
uted so that one part of the nervous system behaves essentially the same
as any other part, showing no polarity in message transmission. Com-
munication within the animal body is slow and inefficient. Nonetheless,
they do exhibit the phenomenon of sensory adaptation or habituation,
the simplest form of learning, in that they cease to capture prey when
they are food-sated.
Fig. 7.5. (A) Longitudinal section of a hydra attached to a substrate, with tentacles ex-
tending upward surrounding the mouth opening. (B) Fine structure of a hydra showing:
(1) sensory cells and (2) epidermal nerve net. [Source: (A) Wikimedia Commons; (B) See
Note 32: Hyman LH. (1940); after Groselji. (1909).]