Tadpoles and aquatic worms
For their gas exchange, tadpoles and aquatic
worms absorb air through their skin and external gills.
The gills are feather-like projections that
produce a large surface area for gas exchange with the water - so
oxygen can be absorbed for cellular respiration and waste carbon
dioxide removed.
With adult amphibians the gas exchange takes
place mainly through their skin and lungs.
Insects
Insects do not have a transport system and
gases cannot be directly exchanged with respiring cell tissue and
the external air.
Insects have tiny holes called spiracles
all along the side of their body.
The spiracles open out into tiny tubes called
trachea - with a moist surface, through which insects pump air
in and out.
The trachea are stiffened to prevent the
minute 'tubes' collapsing.
The trachea have many minute branches called
tracheoles which connect to cells - this increases surface area and
shortens gas diffusion distance and time.
At the end of the trachea is a tiny drop of
water that connects it to the cells.
So, gases can diffuse through the trachea and
water into the cells - providing the cells with oxygen for
respiration.
The spiracles can close to prevent evaporation
and keep the exchange surfaces moist.
Slugs
The slug absorbs air through its skin and has
a moderately large surface area to volume ratio.
The gas exchange surface is moist to dissolve
gases which can diffuse through the moist interface.
The skin membrane is thin to give a short diffusion time.
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