Kiseijuu: Sei no Kakuritsu

Mysterious creatures have colonized Earth by taking possession of some inhabitants. A high school student, Shin'ichi, has been infected, but his brain is spared. Only his right arm is possessed by a parasite, Migi, which he gradually gets to know. Both must now cohabitate, but at the same time, Shin'ichi understands that the proliferation of parasites ultimately threatens the human species.