SECOND SUB-KINGDOM. 



AETIOULATA. 



The Articulata — incomparably the most numerous division of the Ani- 

 mal Kingdom — include all the Invertebrates having jointed bodies. The 

 skeleton is external, the skin hardened in most cases into a bony crust, 

 affording the necessary fulcra to the muscles of locomotion. This skele- 

 ton is sometimes in the form of successive rings; these, in the higher 

 Orders, are soldered together, so that only two or three divisions, or one 

 solid shield, can be discovered. The jaws, when present, are lateral. 

 " The most minute comparison (says Agassiz) does not lead to the dis- 

 covery of a distinct plan of structure uniting all these animals into one 

 natural primary group. What holds them together and keeps them at a 

 distance from other groups, is not a common plan of structure, but a 

 greater simplicity in their organization." 



The Palseontological history of the Articulata is much behind that of 

 the other divisions of the Animal Kingdom. So universally distributed 

 and numerically abundant at the present day, they are least perfectly 

 represented among the relics of a former world. Their manifold, com- 

 plex organization, which in the recent state fits them so admirably for 

 generic and specific comparisons, is fatal to their entire preservation, 

 and the fossil examples are often so fragmentary as to admit of little 

 more than the determination of their Class and Family. The number 

 catalogued forms but a small proportion of those which have probably 

 existed. Bronn enumerates 1551 fossil Insects, 131 Arachnoids, 891 

 Crustaceans, and 292 Annellids. Representations of each Class are 

 found in the Palaeozoic rocks. 



