This question is asked often, so I thought I would put up a proper answer to link to.
Earthworms and composting worms are different sub-species of worms.
Earthworms come from the anecic or endogeic group – they make horizontal and lateral burrows, up to 3 to 4 feet deep.
They need this burrow system to draw their food down into so it can decompose, returning to the surface to drag more in and leave their castings (manure) on top.
Composting worms come from the epegeic group – they do not make burrows, and live in manure or leaf litter in the wild, for instance.
Earthworms arent suitable in a contained farm as they build burrows; the composting farm destroys these.
They are also long-lived, and long to mature, taking 2 to 5 years to become sexually mature and they lay a lot less cocoons compared to shorter lived composting worms.
They are not SUSTAINABLE in a contained farm. At best they may survive, but usually will not prosper.
Composting worms, on the other hand are shorter lived; breed quickly and eat a lot of material. They make a lot of VC and it may be of better quality than VC from earthworms (but earthworm VC is still beneficial).
Only about 6 species of worms, out of 3000+species are suited to composting and contained farms.
The species which are suited to composting are:
– Red wiggler/tiger worms (E.fetida/andrei)
– Blue worms (P.excavatus/spenceralia)
– What are also called red worms (L.rubellus)
– European night crawlers (E.hortensis)
– African Night Crawlers (Eudrilus euginae)