civ Introduction. 



Sub-kingdom, Arthropoda. 



Animals consisting of a series of more or less heteronomous 

 segments, the ' Metameren ' or ' Folgestiicke "" of Haeckel, to which 

 jointed appendages are articulated ventrally in pairs, and, ordi- 

 narily, in very different proportions and grades of development in 

 the different regions of the body. These appendages are, in con- 

 tradistinction to those of Vermes, hollow, and have muscles pro- 

 longed into their interior. This external integument is rendered 

 more or less rigid by chitinous deposit, which may be made still 

 more resistent by calcification. Chitinization extends itself from 

 the exterior into the interior, and in many cases an endophragmal 

 skeleton is thus formed, which arches over the ventrally-placed 

 portion of the nervous system. Tubular prolongations of the cuti- 

 cular chitine extend inwards along the various ducts and canals 

 opening on to the exterior, and are shed together with the inte- 

 gument in the moultings which these animals, so long as they 

 continue to grow, must necessarily go through. 



The chitinous deposit at the commencement of the digestive canal 

 may take the shape of ' lips,' or even of non-segmented processes, 

 like the 'jaws' of certain Vermes, but the true functional jaws of 

 Arthropoda are always produced by the modification of the hollow 

 segmented aj)pendages of more or fewer of the anterior segments 

 of the body. The organs of special sense are ordinarily confined to 

 the prae-oral segments, and, like the rest of the appendicular skeleton 

 with which they are, either actually or morphologically, connected, 

 contribute at least as much to the heteronomy of the external 

 appearance as the segments upon which they are carried. 



The antennae, e3^es, and auditory organs are all but invariably 

 limited to the prae-oral cephalic segments, and the muscles of the 

 chitinous elements of the segments are never found to form con- 

 tinuous antero-posterior layers, as in Vermes, corresponding with 

 the length and often with the circumference of the body. On the 

 other hand, the chief internally-placed system of animal life, the 

 gangliated nerve-cord, presents a striking resemblance to the homo- 

 logous system in Vermes; and the respiratory system of certain 

 Myriopoda appears to attain in its stigmata an approximation to 



