NATURAL HISTORY OF ARTHROPODS. 



BRANCH VII.- -ARTHROPOD A. 



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 arrange- 

 ment 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 attach- 

 ment 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 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 (a^jjo*', 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. 



The Arthropoda have the following features in common, some also being common 

 to many worms : The body is (except in a few forms, the result of adaptation), bilater- 

 ally symmetrical, one side being a repetition of the other ; and is made up of a varying 

 number of rings (called segments, somites, or arthromeres) arranged one after another, 

 and each ring theoretically bearing a pair of appendages, which in turn are jointed to 



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VOL. II. 1 



