In considering transitions of organs, it is so important to bear in mind the probability of conversion from one function to another, that I will give another instance.

Pedunculated cirripedes have two minute folds of skin, called by me the ovigerous frena, which serve, through the means of a sticky secretion, to retain the eggs until they are hatched within the sack.

These cirripedes have no branchiae, the whole surface of the body and of the sack, together with the small frena, serving for respiration.