In invertebrates, specialized sense
organs for monitoring gravity and lowfrequency
vibrations often appear as
statocysts. Each is a simple sac lined
with hair cells and containing a heavy
calcareous structure, the statolith (Figure
35-28). The delicate, hairlike filaments
of sensory cells are activated by
the shifting position of the statolith
when the animal changes position.
Statocysts are found in many invertebrate
phyla from radiates to arthropods.
All are built on similar principles.
The vertebrate organ of equilibrium
is the labyrinth. It consists of
two small chambers (saccule and
utricle) and three semicircular
(Figure 35-25B). The utricle
and saccule are static balance organs
that, like invertebrate statocysts, give
information about the position of the
head or body with respect to the
force of gravity. As the head is tilted
in one direction or another, stony
accretions press on different groups
of hair cells; these cells send nerve
impulses to the brain, which interprets
this information with reference
to head position.
|Figure 35-28 Types of statocysts, static balance organs of invertebrates. A, Statocyst of the medusa of the
hydrozoan Obelia. B, Statocyst of the bivalve mollusc Pecten.
The semicircular canals of vertebrates
are designed to respond to rotational
and are relatively
insensitive to linear acceleration. The
three semicircular canals are at rightangles to each other, one for each axis
of rotation. They are filled with fluid
(endolymph), and within each canal is
a bulblike enlargement, the ampulla,
which contains hair cells. The hair cells
are embedded in a gelatinous membrane,
the cupula, which projects into
the fluid. The cupula is similar in structure
to the cupula of the lateral line
system of fishes. When the
head rotates, fluid in the canal at first
tends not to move because of inertia.
Since the cupula is attached, its free
end is pulled in the direction opposite
the direction of rotation (Figure 35-29).
Bending of the cupula distorts and
excites the hair cells embedded in it,
and this stimulation increases the discharge
rate over afferent nerve fibers
leading from the ampulla to the brain.
This increased discharge rate produces
the sensation of rotation. Since the
three canals of each ear are in different
planes, acceleration in any direction
stimulates at least one ampulla.
|Figure 35-29 How the semicircular canals respond to angular acceleration. Because of inertia, endolymph in the semicircular canal corresponding to the plane of motion
moves past the cupula in a direction opposite to that of angular acceleration. Movement of the cupula stimulates hair cells.