214 HOMOGANGLIATA. 



and activity to be capable of progression upon land ; neither are any of 

 them furnished with organs of sense, that must be indispensable for the 

 security of creatures exposed to those innumerable accidents to which 

 the inhabitants of a rarer element are perpetually liable. 



(556.) The type of structure met with in the HOMOGANGLIATA admits 

 of far higher attributes and allows the enjoyment of a more extended 

 sphere of existence : senses become developed proportionate to the in- 

 creased perfection of the animal; limbs are provided endowed with 

 strength and energy commensurate with the development of the nervous 

 ganglia that direct and control their movements; and instincts are 

 manifested in relation with the increased capabilities and more exalted 

 powers of the various classes as they gradually rise above each other in 

 the scale of animal development. 



(557.) The most obvious though not the most constant character 

 that distinguishes the creatures we are now about to describe is met 

 with in their external conformation : they are all of them composed of 

 a succession of rings formed by the skin, or 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 lowest forms of the ARTICULATA, the body is 

 extremely elongated, and the rings proportionately numerous ; the in- 

 tegument, moreover, is soft and yielding, and, as a necessary conse- 

 quence, the limbs appended to the diiferent segments are feeble and 

 imperfect : such is the structure met with in the Worms, or ANNELIDANS, 

 properly so called. 



(558.) As we advance, we perceive the tegumentary rings to become 

 less numerous, and the skin of a denser and firmer 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, and the instruments of sight and touch begin to assume 

 considerable perfection of structure. This state of development we find 

 in the MYRIAPODA, or Centipedes. 



(559.) In the INSECTS, the concentration of the external skeleton is 

 still more remarkable. 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 together in those parts where great strength 

 and firmness are necessary, and scarcely any traces are left to indicate 

 their existence as separate pieces ; so that, instead of exhibiting that 

 succession of similar segments seen in the Centipede, the body is appa- 

 rently divided into three distinct portions : viz. the head, that contains 

 the organs of the senses and the parts of the mouth ; the thorax, sus- 

 taining the limbs, or instruments of progression ; and the abdomen, en- 

 closing the viscera subservient to nutrition and reproduction. 



(560.) In a fourth division of articulated animals, namely the 



