I have three kinds of these "flat worms" in my collection. They are found under partly submerged stones.
Nemertinea
There are seven different nemertines in the collection. They are recorded from holdfasts, seaweed tangles and from among vermetus tubes.
Nematoda
There are two or three different marine nematodes in the collection. They are most common in the finer moss.
Sipunculoidea
There are two kinds of sipunculids, which seem quite distinct. Taken from eel-grass roots, from under rocks and mussels.
The specimens were identified from the following papers:
1899
Fewkes, J. W.