Most suitable package
1- In etanol 100% and etanol 50%.
Simply place the samples in a glass or plastic recipient full of the water in which they were collected and be sure not to let much air -the smallest amount possible- and to close it tightly. Use adhesive tape or parafilm if needed or available.
2- Living samples
To get the optimal keep the samples in glass or plastic recipients cooled, in the same water their were sampled, with some air inside. Best way is to use some old clothes or paper as an envelope and place it all in a thermal bag with some ice inside. That way they'll keep cold though not frozen -they would die.
Information required from sampling site
As important as the planarians themselves, is the exact location of the sampling site. The name of the river, beach or lake if available, and/or the closest road kilometric mark and/or the name of the closest village. Even better would be the use of a GPS or the UTM system. The more complete the description of the site, the better. It would also be worth of to describe the sampling site vegetation and abundance of other living forms.