HOMOGANGLIATA. 185 



outward integument, which from its hardness constitutes a kind 

 of external skeleton, supporting the body, and giving insertion to 

 the muscles provided for the movements of the animal. In the 

 class CIRRHOPODA alone is this external characteristic wanting, 

 and the Homogangliate organization masked by a tegumentary tes- 

 taceous coat of mail, which they seem to have borrowed from the 

 molluscous type. In the lowest forms of the ARTICULATA the 

 body is extremely elongated, and the rings proportionately nu- 

 merous ; the integument moreover is soft and yielding, and, as a 

 necessary consequence, the limbs appended to the different seg- 

 ments are feeble and imperfect : such is the structure met with in 

 the worms, or ANNELIDANS, properly so called. 



As we advance, we perceive the tegumentary rings to become 

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

 adapted to support the action of stronger and more powerful 

 muscles ; the limbs likewise become more elaborately formed, their 

 movements more free and energetic, and the instruments of sight 

 and touch begin to assume considerable perfection of structure. 

 This state of developement we find in the MYRIAPODA or 

 Centipedes. 



In the INSECTS the concentration of the external skeleton is 

 still more remarkable, and the integument assumes a hardness 

 and solidity proportioned to the vigorous movements of which 

 the limbs are now capable ; the rings or segments of the body, 

 hitherto distinct, become more or less firmly soldered toge- 

 ther in those parts where the greatest strength and firmness are 

 necessary, and scarcely any traces are left to indicate their ex- 

 istence as separate pieces ; so that, instead of exhibiting that 

 succession of similar segments seen in the Centipede, the body 

 is apparently divided into three distinct portions, viz. the head, 

 which contains the organs of the senses and the parts of the 

 mouth ; the thorax, sustaining the limbs or instruments of pro- 

 gression ; and the abdomen, enclosing the viscera subservient to 

 nutrition and reproduction. 



In the fourth division of articulated animals, namely the 

 ARACHNIDANS or Spiders, a still greater consolidation of the 

 external skeleton is visible ; for in them even the separation 

 between the head and the thorax is obliterated, and it is in the 

 abdomen only that the segments of the body are recognisable. 



Lastly, in the CRUSTACEANS we have various modifications of 

 the outward skeleton adapted to the habits of the different tribes ; 



