Re: Why do fish have backbones?

r norman <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
> 
> The flexible but incompressible rod called the notochord allowed early
> chordates to swim by flapping their tails from side to side.  That was
> a very effective and efficient method of locomotion.  There are other
> ways of swimming rapidly, but that turned out to be a very good one.
> 
> If you want to move efficiently, you have to contract muscles and
> still control the form and shape of the body.  Without some type of
> skeleton, contracting longitudinal muscles on the side of the body
> will simply shorten the body, not bend it.  Hydrostatic skeletons have
> severe limitations so the best way to maintain body shape is with some
> form of skeleton, internal or external.  There other limitations on
> external skeletons.  You find an enormous range of aquatic animals
> alive today using all three strategies -- hydrostatic skeleton in the
> jellyfish, a wide variety of "worms", and molluscs like squid and
> octopus, external skeletons in the crustacea, and internal skeletons
> in the vertebrates.  The vertebrates "won" in being able to produce
> very large very fast moving organsms but the alternatives are still
> extremely effective especially in smaller body sizes.
> 
> Still the notochord is not a backbone (a vertebral column).  The
> vertrebra (plus skull)  were protective structures enclosing the
> central nervous system and, originally, the dorsal aorta.  Only later
> in evolutionary terms did the vertebra replace the notochord as the
> main structural and support element maintaining the body shape. You
> can't consider "modern" vertebrates since everything from most fish on
> up have bone, a radical change in the ability to build skeletons.  To
> see the change from relying on notochord to using the vertebral
> skeleton, you have to look at modern lampreys and hagfish (who retain
> a notochord) and sharks and other cartilaginous fish (whose notochord
> remains but in greatly reduced form).  The change might be due to
> different methods of constructiion.  The notochord is all one piece
> and relies on flexibility to function.  The vertebra are constructed
> segmentally, one in each segment or myotome of the developing chordate
> embryo.  The separate pieces could each become relatively rigid, hence
> very strong, and yet retain flexibility in the way they connect
> together.  In this way, the vertebral column was a much better
> skeletal structure than the notochord. 
> 
> Note: we still have remnants of the notochord in the soft, squishy
> centers of our intervertebral disks.

Thinking about it a bit more, I tend to get an idea of some of the
forces involved.

If you had a theoretical worm with fins, that had no internal
structural skeleton, you could ultimately still generate movement
through the following set of actions:

1. Extension of one half of the fins outward on both sides to increase
drag or contact with the water.  Retraction of the other half of the
fins.

2. Contraction of body segments so that retracted fins move near to
location of extended fins.

3. Extension of Retracted fins and Retraction of Extended fins.

4. Contraction of body segments so that new retracted move to new
extended.

Thinking about it a  bit more, there might be a simpler way: 

To have two or a small number of fins, and then have them go through
single strokes on both sides to move the organism along, without
having a large number of them.

It seems to me that in many ways, the dynamics of walking are simpler
than those of swimming, because at least the force output, tends to be
located in one place, at the points where the organism's feet are
making contact with the ground.

When it comes to swimming, the organism has to create a situation
where high friction with the surrounding water medium is favored. 
Then it has to impart a force to the water in that environment.  Then
it has to change its shape so that it can glide through the water with
minimum resistance, and then remodify itself again so that it generate
more propulsion.

Another dynamic problem of early organisms is one that does not seem
to exist to as great an extent in engineering.  Simply, the ability of
jelly-like masses to impart force into water around them without
deformation.

Engineers can build propellers from at least somewhat solid bodies. 
If you tried to build a propeller from a wet noodle, however, its
effectiveness in imparting force to water or air might be compromised.

Now if an organism were to have a fin, and that fin completely
deformed when it went through the water, it might not be able to
impart as much of a force near to the end of the stroke, if it were
not rigid, in comparison with a stronger fin.

If you could make an organism extend itself sideways, however, the
sideways direction of the organism could act as a substitue for a fin.
 The organism's sideways direction could make it have increased water
resistance at the bend.  If the muscles contracted into the bend, then
they could thus impart force into the water.

A swimming eel could have many different bends down its vertebral
column at the same time.  If the organism could not resist lateral
compression at the bend, however, the limitations in its own internal
structure might limit the amount of force that the organism might be
able to impart into the water.

Thus internal structural resistance to deformation, might have
initially been a way of increasing the capability of producing force
output into the water, and thus increasing the potential speed of
swimming, when needed.

It was only later that this capablity became useful, toward properties
enabling organisms to walk on land.