At least that suggests first pages of this wonderful horror-sf manga by Hitosi Iwaaki. Titular parasytes are dropped in eggs to ground as worm-like living things that search for suitable host. They eat part of host and became it – usually head. And then they eat other humans, driven by killer instinct. Only Hitosi knows what it have anything to do with ecology.
Don’t worry, this is not spoiler, we get to know all of it in very first chapter. Premise sounds good? Enter your usual high school student, Shinji. Worm tried to infect him and failed – became his hand instead (later named Migo and way smarter than his host). Cue wacky adventures like meeting, talking and fighting with other parasytes. Oh, fights. They are done very nicely, even if few moments feel forced. But usually parasytes will attack in most logical way – as they can morph their bodies, creating sharp things from itself to dismember and hole up bodies is most effective way. As bonus it causes a lot of body horror, both in horribly mutilated victims and parasytes themself. So much eyes, so much teeth.
While most parasytes are just murderous serial killers (on par of one human psycho featured in series), some of them developed more complicated personality and outlook on life, despite being emotionless. Main example is of course Migo, coolest hand in manga ever. Or teacher that adjusted to society (mostly), searched for purpose in life and even had child (unquestionably belonging to homo sapiens species). She still eats humans, though.
Very interesting theme appears in this work – monster became more human, human more monstrous. Saya no Uta had same theme, only more horrific. Relationship between Migo and Shinji, parasyte and human went long way. From threatening to cripple host if he say anything and planning to use his classmates as human shields in fight, they end up as experienced, dangerous team that took down a lot of parasytes – not without losses, however.
Overall, I enjoyed this story. It has all things good story needs: interesting premise, main hero (that can be easily identified with) whom took level in badass, cool fights, tragedy, girl(s) and dozen of horrible monsters to get rid of (only it gets more complicated, but let’s not spoil).
ADDENUM: It seems they started making anime based on this manga. Now we all should pray to Elder Gods that they do not fuck it up.