ARTICULATED ANIMALS. 



59 



CHAPTER IX. 



SECOND DIVISIQN OF INVERTEBRATE ANIMALS. 

 ARTICULATED- ANIMALS. 



WE have now arrived at the second great division of the 

 animal creation, which. includes a vast assemblage of crea- 

 tures adapted to exist under a far greater diversity of circum- 

 stances than those we have as yet had an opportunity of examining. 

 The most obvious character by which they are distinguished is 

 met with in their exterior conformation. They are composed of 

 a succession of rings, formed by the skin or outward integument, 

 which, from its hardness, constitutes a sort of external skeleton. In 

 the lowest forms the body is extremely elongated, the segments 

 proportionately soft and numerous, and, as a necessary conse- 

 quence, limbs either do not exist, or are feeble and imperfect. 

 Such is the structure met with in the Annelida or Worms, as, 

 for example, in the Leech (Fig. 53). 



FIG. 53. LEECH. 



As we advance, we find the tegumentary rings become less 

 numerous, and the skin of a denser and more firm texture, adapted 

 to sustain the action of stronger and more powerful muscles ; the 

 limbs likewise become more elaborately formed, their movements 

 more free and energetic. Moreover, the instruments of sight and 

 touch begin to assume considerable perfection of structure. This 

 state of development we find in the Myriapoda or Centipedes 

 (Fig. 55). 



In the Insects the perfection of the external skeleton is still 

 more remarkable, and the integument acquires a hardness and 

 solidity proportioned to the vigorous movements of which the 

 limbs are now capable. The rings of the body, hitherto distinct, 

 become more or less soldered together in those parts where the 



* Articulatus, jointed. 



