If you are looking for the verse form of the poem, it is not a closed form in the sense of a villanelle, sestina, sonnet, and the like. Hardy, an early proponent of blank verse, preferred the rhythm of the poem to it's form. He considered rigid adherence to beat bad poetry.

Hardy says the following lines are partly made up, partly remembered from a Wessex folk-rhyme. He called the form (or lack thereof) in this poem cunning irregularity.