Biology

For those who are not familiar with the story of a conflict between the two horns of a snail, you may like to refer to Zhuangzi (莊子) [則陽篇].

Yes, the idea of this little story is simple. Nonetheless, the war that Zhuangzi observed might have been a genuine biological phenomenon.

It goes like this: Leucochloridium is a group of flatworm parasite (i.e. platyhelminthes) that uses various worm-eating birds as the permanent host. The remarkable bit of its life cycle is, however, most of its species use snails as the intermediate host. When a suitable snail is infected, the parasite would grow and move to the tentacles (to we layman, the “horn”; to biologists, those are actually eyes of the mollusk). 

Depending on the actual species of the Leucochloridium, sometimes only one, but usually both of the tentacles are infected. Once the two light-sensing organs are filled with parasites, it shows off funny angry-looking colors – just like they are in war with each other. More importantly, the snail becomes blind and loses its natural tendency of avoiding light; it moves to the top of a branch, and is easily spotted by birds around.

Yes, you guess that much. To those flying creatures, the colorful light-sensing organ on the snail is no different from a worm. The infected tentacles are quickly nipped away by hungry beaks, and, hurray, the life cycle of Leucochloridium completes.