Question

Earthworm are bisexual still the self fetilisation is not possible in them

Answer 1

Inspite of the fact earthworms are bisexual(have both male and female genitals) they cannot reproduce by self fertilisation. This is because of the positioning of male and female sex organs in different segments and not close to each other.

Answer 2

Earthworms are protandrous, i.e., the testes mature earlier than the ovaries. Self-fertilisation  is not possible in them because of the relative position of the openings of male and female reproductive organs.