But since it is recognized that organic structure is of all considerations the most essential as a guide in a methodical and natural distribution of animals, as well as in determining the true relations between them, it follows that the crustaceans, which breathe solely by means of gills in the same way as molluscs, and like them have a muscular heart, ought to be placed immediately after them, before the arachnids and the insects, which do not have a like organic structure.