Fritz Muller, in order to test the conclusions arrived at in this volume, has followed out with much care a nearly similar line of argument.

Several families of crustaceans include a few species, possessing an air-breathing apparatus and fitted to live out of the water.

In two of these families, which were more especially examined by Muller, and which are nearly related to each other, the species agree most closely in all important characters: namely in their sense organs, circulating systems, in the position of the tufts of hair within their complex stomachs, and lastly in the whole structure of the water-breathing branchiae, even to the microscopical hooks by which they are cleansed.