Alan, I was thinking like you, that it was an Aglajid. In line with Leslie's identification of these guys as acoel flatworms, here is Dr. Rudman's response from the Sea Slug Forum: http://www.seaslugforum.net/display.cfm?id=15761
"Dear Nick,
These are acoel flatworms. Not as colourful as the usually large polyclad flatworms, but the acoels often occur in very large numbers. I don't know if it is just coincidence that they have a forked tail, which looks very similar to that of Chelidonura, a genus of opisthobranchs which feed on them.
Your animal is probably the same species as the one in Atsushi Ono's message [#974]. Like most flatworms they move much faster than most sea slugs.
Best wishes,
Bill Rudman"
"Dear Nick,
These are acoel flatworms. Not as colourful as the usually large polyclad flatworms, but the acoels often occur in very large numbers. I don't know if it is just coincidence that they have a forked tail, which looks very similar to that of Chelidonura, a genus of opisthobranchs which feed on them.
Your animal is probably the same species as the one in Atsushi Ono's message [#974]. Like most flatworms they move much faster than most sea slugs.
Best wishes,
Bill Rudman"