And not only the sperm (although here you can see some amazing sperm movies, and access the pdf)...
The most evident structure of macrostomids is perhaps the stylet; its varied and curious morphology posesses enough variability to serve as species identifier (the fact that this is not always rightly so is part of a different story). But when looking carefully we can't but be amazed at the complexity of other organs and cells involved in reproduction: the sperm, the female antrum, the prostate secretions. And, of course, at the misleading harmony intricacy of the reproductive behaviour:
But where does all this variation and complexity come from? Why is it there? We think the answers probably dwell in the conflicts between mating partners and between sexual competitors. Although engaged in the common task of producing offspring, two mating partners are seldom united in their interests: each would like its gametes to be best served by the other. And the competition for mates can also lead to plenty of disagreements...
In this frame of mind, we formulate hypotheses for the evolution of different traits of the reproductive organs, sperm and behaviour, using our model species Macrostomum lignano. With the aid of light microscopy, TEM, and many hours of observation (and discussion) we look for processes hinting at male-female interactions ranging from behavioural to chemical. We further propose a fertilization mechanism, based on the ultrastructure of the sperm cell and the positioning of the sperm in the antrum.
Dita B. Vizoso, Gunde Rieger, & Lukas Schärer, 2010. Goings-on inside a worm: functional hypotheses derived from sexual conflict thinking. Biological Journal of the Linnean Society 99: 370-383 [online abstract]
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