What is this worm-like thing??

Question
QUESTION: I've had my saltwater tank for over a year now, and yes I have a couple live rocks.  Last Friday I happened to wake up earlier than usual and noticed this worm-like "thing" swimming spirals in my aquarium that I've never seen before.  It was lightning fast, and moved all around, only swimming in 1.5 inch diameter circles.  It was about 3 inches long, and just less than half an inch wide if I had to guess... what is it please?  Thanks for your help!

ANSWER: Hi Dipali. I am going to need a better description of the worm if possible. Color? Smooth body or centipede like? Legs present or none? Striped? Flattened or cylindrical body shape? Lots of worms are not well suited for swimming and will use the only means they have and that is by spiraling through the tank. Even your common bristleworm will use this sort of locomotion if they are in a hurry to get out of site. If you could get back to me with a little more description I would be glad to further help you in identifying this mysterious creature!

---------- FOLLOW-UP ----------

QUESTION: Hello again, Jennifer!

Okay... this worm-like thing seemed more flat than round, but it was not paper-thin.  It was swimming REALLY fast in small circles like I described above (I thought it was my grouper going crazy for a second until I looked closer).  I have a 35 gal. octagonal tank which is longer than it is wide, and it was swimming all the way from the top to the bottom, and all around the sides.  I didn't turn the lights on, so it did not hide but just kept swimming.  It was peach in color, lighter in the middle, maybe darker orange at the sides.  It seemed to have a smooth body-type, I didn't see any texture to it otherwise.  I WISH I could've taken a picture or some video, but it was so fast.

Alright I hope that helps us out!!  Can't wait to learn what it is... and if it's safe for my fishes.  Thanks again for your help!

PS: I think this is unrelated, but I have bristleworms that are very microscopic and only really visible in the dark (nothing like the worm I'm talking about -- but I figured that I'd mention it)

Answer
Hello again. I think I may have an identification on your alien worm. Most likely it is a worm from the phylum Nemertea. Also known as ribbon worms. They look like long strips of ribbon that can be anywhere from 1/2 cm in length to 100 ft in length! They have a flattened outer surface which is ciliated for locomotion however there are no obvious landmarks or appendages on their body. Most of these worms are a buff to brown color with an occasional specimen being black or brightly colored. Identification can not be dependant upon color because there are lots of variations in each species due to area it is collected from, sex, and what kinds of food it feeds upon. They are burrowers that inhabit the substrate and some are pretty good swimmers using rapid whirling undulations of the body. These are mostly predatory worms and a few are scavengers. To learn a little more on these worms I have written the name of a website that can give you a little more info than I can on these worms. http://www.reefkeeping.com/issues/2004-01/rs/index.php          
I hope this helped.