Here's what the intricacies are for:
Lukas Schärer at the University of Basel in Switzerland and his colleagues watched 16 species of promiscuous hermaphroditic flatworm (Macrostomum), which have a variety of sperm shapes (pictured), mating under a microscope. After sex, some species suck out the ejaculate, possibly as a way of selecting which sperm are ultimately accepted.
The researchers found that those species that exhibit this sucking behaviour have ornate sperm with features such as a pair of long bristles emerging at the mid-point and a tail resembling a paint brush. These appendages can become lodged in the female orifice after copulation, preventing the sperm from being sucked out. Species that don't remove sperm have evolved simpler sperm that tend to be smaller and lack hairs or bristles.
I seem to remember that certain types of porcupines have barbed quills that are meant to cause additional damage when pulled out, but I can't find a reference for this. Looking this up -- and trying to find the term for arrows/darts that are meant to be hard to pull out -- took me on a ramble through Wikipedia that introduced me to the concept of "fletching" on arrows. Which, strangely enough, appears to be a common-ish variant of the Urbandictionary term "felching." Language prefigures science as it were.
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