About three years since I presented to the notice of the Royal Society a few observations on the genus planaria. From that period to the present having had no opportunity of extending my researches to more than two or three species in addition to those formerly described, I was unwilling to trespass upon the time of the Society by any further remarks, until I had ascertained the remaining species of this genus. A circumstance, however, attending some experiments in which I have been lately engaged, of rather a strange character, forming another interesting feature in the history of these very extraordinary animals, induces me to lay before the Society the present communication.