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Hi,
I think this is an Enchytraeus annelid, I have found many of these so far, some soil-particles are always visible inside them. Also live in decaying wood, etc. I have a photo of the head, it's more like Annelid then Nematode. The rings are not always so visible when the animal is not contracted. Spotting some bristles with microscope would help to decide if your specimen is truly an annelid, but there are some Enchytraeidae without bristles (setae) also. A picture from head:

Best regards,
Walter