The Acephalocyst or Hydatid is composed of a vesicle containing fluid. It propagates by means of internal gemmules, which are developed between the layers of the membrane composing the vesicle, and, after a certain time, are thrown off internally.

The author, after pointing out these distinguishing characters of the Hydatid, referred to the confusion which had arisen, from want of proper observations on this point, and which, with other causes, had been the reason why the animal nature of these creatures had been denied by some writers of great authority.