Which animal has the shortest lifespan?

Depends on whether you consider bacteria to be "animals". If you do, then you will have to consider a wide variety of bacterial species with a life cycle of 15-20 minutes.

If you don't, then a case can alternatively be made for the male of the mite Acarophenax tribolii -- which develops inside the body of its mother, fertilises its female siblings, and then dies before it is even born, all within the space of a couple of hours.

The gastrotrich (a minute aquatic animal) lives the shortest--as short as a three days.

Micro organisms does have the shortest lifespan than in animal kingdom.

But when you're talking about animals. There is one insect called 'Mayfly'. It's lifespan is only ONE day.

Insects have very short lifespans too, Adult mayflies, depending on the species, have life spans ranging from half an hour to a whole day!

example of any animal which can be seen out of the microsope ?

Animals? The chipmunk only lives 3-5 years. A guinea pig maybe 6-10.

I jsut thought you should know that mayflys actually live for an average of 3 years... they start out in a Larva/Nymph stage... and all through that stage they are considered to be living.
Found my answer at: http://simple.wikipedia.org/wiki/Complete_metamorphosis