This group of Articulata was still further divided into three classes : Worms, Crustacea, and Insects.

This classification was long prevalent, and even at the present time it is found in use in a few text-books, though when naturalists came to study more thoroughly the principles upon which animals should be grouped, and especially upon applying the rev- elations of embryology, it was seen that the class of Worms contained the most hetero- geneous elements, and that while certain members of it were possibly closely related to Crustacea and Insects, the great majority had no such affinity, and that the features uniting them were of not so much importance as many others.

Hence, as we have seen in the preceding volume, the group of Articulata has been dismembered and dropped from use, and even the class of worms is far from being a natural one.

According to the majority of the naturalists of the present day, the Crustacea and the Insects are together considered as forming a sub-kingdom, ARTHROPODA (ajjo, a joint, and novg, 71066$, a foot), but the tendency of scientific thought at the present time is toward the discarding of this group, and toward the belief that the Crustacea and the Insects are generically no more closely related to each other than they are to the worms, and that each should be raised to the dignity of branches.

The reasons for such a course are many, but for convenience, in the present work, the prevailing classi- fication will be retained.