Poem structure for cloud zoo

What is the poem structure of "Cloud Zoo" by Norma Gillet?

CLOUD ZOO

I thought I saw a cloud whale
Sail through a sea of blue;
It changed into an elephant
Of most unusual hue,
And even as I marvelled,
It split itself in two.

I watched a long white serpent
Winding its cloudy way,
To pounce upon a cloud frog
That unsuspecting lay.
The little frog became a bird
And slowly flew away.

All afternoon I watched them,
Such magic as they knew!
I saw a white rhinoceros,
And white flamingos, too,
Till evening shut her deep blue tent
Over my private zoo.
[Norma Gillett]