Together with Martin Oeggerli we have made some SEM images of sperm and Martin them arranged an colored them:
We can see that the larger sperm of M. lignano has long protrusions called sperm bristles, while the smaller sperm of M. pusillum does not have them.
Our research indicates that these bristles are an adaptation to coevolution between sperm-donors and sperm-recipients. Indeed, we found that Macrostomum species have lost these bristles many times and these losses are correlated with a change from reciprocal to hypodermic mating.