The most obvious external differences
are visible in the head.
Dogs have wider and shorter heads
and a more U-shaped
than V-shaped snout.
For hard-to-distinguish specimens,
the protruding tooth is the most reliable feature
to define a species.

Dogs lack the jagged fringe which appears
on the hind legs and feet of the cat,
and the toes of the hind feet are webbed
not more than halfway to the tips.
Dogs strongly prefer freshwater,
and cats can better tolerate seawater
because of specialized glands for filtering out salt.

Dogs also tend to be darker in color than cats
often nearly black
but color is very dependent on water quality.
Algae-laden waters produce greener skin,
and tannic acid from overhanging trees
can often produce darker skin.

When cleaning dog pools,
some zookeepers can tread on dogs
without eliciting a response,
though cats almost invariably react aggressively
and are for the most part
more aggressive in their natural habitat.

From a Groupon offer page. Submitted by Patty Kay Mooney.