Earthworms can be kept for months in a wormery simply because we provide them with food and they are able to breath because the material is damp.
Lug and Rag exist in a marine environment. Generally speaking we keep them in newspaper simply because it absorbs moisture which inevitably the worm excretes. If it lies about in this it would rapidly die. Thus worms in sea water must have a large amount of it in which to swim and it soon becomes polluted and must be changed. Think about the aeration that must happen in normal aquaria. Every year I used to take Kids from Telford to Anglesey for the day to catch a really good Spring tide and study the shore. We took a limited selection of flora and fauna back with the idea that they would be returned after three weeks being writ large. They were installed in carefully controlled aquaria (aeration and temperature being paramount) Crabs, shrimps, prawns, limpets, winkles and Whelks 'performed' well, and anemones were also useful if needing more care, but try as I might I could not keep worms alive.
However I always take worms back after fishing and they keep perfectly well in newspaper placed in an insulated bait box with one freezer block!! Vermiculate is probably better than newspaper (even the Guardian) but more expensive. I've never had any success keeping them in the fridge, but perhaps ours is turned down too low. I heartily agree with the notion that it's the state at which they are dug/bought that is important in terms of how long they last.
Rag will live for quite a while in a bucket covered in plenty of water and a aerator with a air stone on the end, change the water when its dirty and remove any that die, this way they will live for about a month.I put lug in catlitter trays with just enough water to cover them they have lasted a couple of weeks no probs