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Re: Breathing



Stanley Friesen wrote:

> >although I remain curious about the evolutionary and physiological
> factors
> >that apparently *precluded* (?) in terrestrial vertebrates the
> true
> >counterflow respiratory systems (aka gills) seen in more primitive
> >vertebrates.
> 
> It is likely a combination of excessive drying in exposed gills and
> a tendency of gills to collapse (loosing surface area) when not
> supported by water.

I'm sure these were the immediate reasons.  But most fish have
substantial support for their gills, and I would imagine it would be
quite possible to evolve a gill which was more suited to air
breathing.  Crabs and related crustaceans have done this more than
once.

But why impair the effectiveness of the gill in water, when you
already have an air-breathing lung?  Early tetrapods continued to use
their gills in water, although I guess they developed larger lungs to
spend longer on the land.  Amniotes dispensed with their gills much
later, when they became more terrestrial.

> >>>The oldest known jawed vertebrates had *both* structures! <<
> (I added the qualifier mostly because I am unsure of he condition
> in
> agnathans).

As far as I know, lungs are known only from crown-group osteicthyans
(is that how you spell it?), and not from elasmobranchs or any
agnathan.

IMO the best terrestrial gas exchange system is the much-maligned
tracheal system of insects.  In larger insects the flow is mostly
unidirectional.  The tracheal system can deliver more oxygen than the
vertebrate bloodstream, allowing higher metabolic rates.  And it does
so with a minimum of water loss, allowing insects to live in the
driest conditions.  It is also highly efficient, because air is much
less viscous than blood.

Bill
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