Ch. 5

Question 1

functional definition of the word skeleton is

Answer 1

A solid or fluid system permitting muscles to be stretched back to their original length following a contraction.

Such a system may or may not have protective and supportive functions as well.

Question 2

A skeletal system is essential simply because

Answer 2

muscles are capable of only two of the three activities required for repeated movements: Muscles can shorten or relax, but they cannot actively extend themselves.

the muscles antagonize (act against) each other, making controlled, repeatable movement possible.

Question 3

Environmental differences for skeletons

Answer 3

once! In terrestrial environments, a rigid skeletal system is essential, in part because the skeleton must also serve to support the body in a nonsupportive medium.

On the other hand, aquatic organisms are supported by the medium in which they live, so they don’t require a rigid skeletal system.

Indeed, many aquatic animals use fluid as the vehicle through which sets of muscles interact.

That is, these animals have hydrostatic skeletons.

Question 4

The basic hydrostatic skeleton requires:

Answer 4

1. the presence of a cavity housing an incompressible fluid that transmits pressure changes uniformly in all directions;
2. that this cavity be surrounded by a flexible outer body membrane, so that the outer body wall can be deformed;
3. that the volume of fluid in the cavity remain constant; and
4. that the animal be capable of forming temporary attachments to the substrate, if progressive locomotion is to occur on or within a substrate.

Debatable?
5. the presence of a deformable but elastic covering or the presence of at least two sets of muscles that can act against each other.

Question 5

Let us assume that these four attributes are met in the hypothetical organism with only longitudinal muscles

Answer 5

This cylindrical being is equipped with longitudinal muscles only.

If this animal attaches at point X and then contracts its musculature, the increase in internal hydrostatic pressure will deform the outer body wall, resulting in a shorter, fatter animal.

This animal can regain its initial shape only if it is surrounded by a stiff, elastic covering that will spring back to its original shape upon relaxation of the longitudinal musculature.

Such a stiff covering could be difficult to deform in the first place and is not commonly encountered among unjointed invertebrates.

Question 6

Let us assume that these four attributes are met in the hypothetical organism with longitudinal and circular muscles

Answer 6

Instead, we add a second set of muscles (circular muscles) to our hypothetical animal.

Forward locomotion then results from the series of contractions.

The circular muscles are contracted, and the longitudinal muscles are stretched.

In (a), The longitudinal muscles now contract while the circular muscles relax, producing the shorter, wider animal (and generating powerful radial forces in the process).

In (b), the animal releases its anterior attachment to the substrate and forms a new temporary attachment posteriorly.

The circular muscles then contract while the longitudinal muscles relax, thrusting the animal’s anterior end forward.

We see that the animal has advanced by a distance d and has regained its initial shape, ready to repeat the cycle of muscular contractions.

From this discussion, it is obvious that one addition must be made to the previous list of requirements for a functional hydrostatic skeleton:

5. the presence of a deformable but elastic covering or the presence of at least two sets of muscles that can act against each other.