Many instances of this worm-like creature were found in a water system for a mountain cabin in the Sierra Nevada mountains in late June / early July. The water system collects water from an enclosed spring. Can you help identify this creature?

Clues:

1. The worms have two "points" on their heads.
2. They can stretch out and be long and thin, or when poked (or are resting) they contract into a small blob.
3. Here is a picture of a single worm with markings shown.
4. Here is a picture of a few worms, somewhat contracted.
5. Here is a movie of them moving, in natural light.
6. Here is a movie of them moving, with backlight.
7. Here are pictures of their undersides and their internal structures.
8. Whenever I pull some out of the water and put them into a tupperware with the same water, they only survive for around 24 hours, after which they seem to disintegrate. Here is a picture of about 20 of the worms after about 24 hours, the bulk of them have halfway-decomposed and form a heap in the upper right. After another day it'll just look like debris in the water, you wouldn't even recognize that there were worms in it. There is a piece of bacon in there because some suggested putting it there to test if they were leeches, but this disintegration happens repeatably and regardless of the bacon; please ignore the bacon.
   
9. Someone suggested that they are trematodes, but there are no snails, frogs, or other creatures in the storage and collection sections of the water system; we have looked very closely. There may be something within a 30-foot section of pipe, but we doubt it.

Can you help us identify this creature? Even just possible families of creatures or general categories would be helpful for us.

Many instances of this worm-like creature were found in a water system for a mountain cabin in the Sierra Nevada mountains of the US, in late June / early July. The water system collects water from an enclosed spring. Can you help identify this creature?

Clues:

1. The worms have two "points" on their heads.
2. They can stretch out and be long and thin, or when poked (or are resting) they contract into a small blob.
3. Here is a picture of a single worm with markings shown.
4. Here is a picture of a few worms, somewhat contracted.
5. Here is a movie of them moving, in natural light.
6. Here is a movie of them moving, with backlight.
7. Here are pictures of their undersides and their internal structures.
8. Whenever I pull some out of the water and put them into a tupperware with the same water, they only survive for around 24 hours, after which they seem to disintegrate. Here is a picture of about 20 of the worms after about 24 hours, the bulk of them have halfway-decomposed and form a heap in the upper right. After another day it'll just look like debris in the water, you wouldn't even recognize that there were worms in it. There is a piece of bacon in there because some suggested putting it there to test if they were leeches, but this disintegration happens repeatably and regardless of the bacon; please ignore the bacon.
   
9. Someone suggested that they are trematodes, but there are no snails, frogs, or other creatures in the storage and collection sections of the water system; we have looked very closely. There may be something within a 30-foot section of pipe, but we doubt it.

Can you help us identify this creature? Even just possible families of creatures or general categories would be helpful for us.