BIO1022 WK 4 NOTES
TXTBK NOTES CHP 37.3
have evolved three types of skeletal system:
hydrostatic skeletons (
skeletal system in which fluid contained within a body cavity is the
A rigid skeletal system that lies external to the animal’s soft tissues),
The bony skeletal system of vertebrate animals, which lies internal to most of
the animal’s soft tissues).
Hydrostatic skeletons support animals by muscles that act on a fluid-filled cavity.
They are found in nearly all multicellular animals as well as in many vascular plants. In animals that
depend on a hydrostatic skeleton,
fluid contained within a body cavity serves as the supportive
component of the skeleton. Muscles exert pressure against the fluid to produce movement.
Two sets of opposing muscles surround the fluid, controlling the width and length of the body cavity
and, in many cases, of the whole animal
Circular muscles reduce the diameter of the body cavity, and longitudinal muscles reduce its length.
sea anemone can bend its body in various directions to feed or to resist water currents by closing its
mouth to keep its fluid volume constant and contracting its circular and longitudinal muscles. Sea
anemones extend themselves into the water column by contracting their circular muscles and relaxing
their longitudinal muscles. Controlling the amount of fluid in their body cavity also allows sea anemones
to adjust their shape. When threatened, a sea anemone rapidly retracts by contracting its longitudinal
muscles and allowing water to escape from its body cavity.
Similarly, earthworms burrow through the soil by moving a series of hydrostatic segments along their
body .By contracting longitudinal muscles in a few segments, they shorten those segments and also
widen them to anchor those segments against the soil. They then extend intervening segments between
anchor points by contracting circular muscles, causing the segments to lengthen and move the animal’s
body through the soil.
Even vertebrate animals with rigid endoskeletons have hydrostatic elements that provide flexibility and
cushion loads transmitted by the skeleton. These include
A fluid-filled support structure found between the bony vertebrae of the
backbone that enables flexibility and provides cushioning of loads.)
A type of connective tissue found, for example, in the walls of intervertebral discs and
the joint surfaces between adjacent bones.)