Excerpt from The Standard Natural History, Vol. 2
Cuvier, in his great, divisions or branches of the animal kingdom, recognized a group Articulata characterized by having a bilaterally symmetrical body, composed of a series of rings or segments serially arranged. An excellent example of the arrangement of these rings can be seen in the common earth or angleworm. These rings make a hardened external skeleton, which at once forms a framework for the attachment of muscles, and also a protection for the internal organs. Typically, there is found in each segment a portion of each of the more important organs of the body. Just under the dorsal surface is found an elongated dorsal vessel, which represents the heart; the intestine lies in the median line of the body, which it usually traverses from end to end, while the nervous system, consisting of a series of enlargements, called ganglia, connected by nervous cords, extends along the floor of the body. 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 revelations of embryology, it was seen that the class of Worms contained the most heterogeneous 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 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 classification will be retained.
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