earthworms and tapeworms are bisexual animals.

a) what technical term is used to describe it ?

b) in what ways do these animals differ in mode of sexual reprodution?

Such organisms having both male and female reproductive organs are called  hermaphrodite. Earthworm and tapeworms are hermaphrodite animals.
 Though Earthworms are bisexual yet they need mate to undergo sexual reproduction. Cases of self fertilisation have also been reported in some species in earthworms. But it is rare and cross fertilisation is common.

In case of tapeworms also, cross fertilisation is common in most of the species and self fertilisation is rare.