FREE BOOKS

Author's List




PREV.   NEXT  
|<   118   119   120   121   122   123   124   125   126   127   128   129   130   131   132   133   134   135   136   137   138   139   140   141   142  
143   144   145   146   147   148   149   150   151   152   153   154   155   156   157   158   159   160   161   162   163   164   165   166   167   >>   >|  
these three classes, the Worms, presents the typical structure of that branch in the most uniform manner, with little individualization of parts. The body is a long cylinder divided through its whole length by movable joints, while the head is indicated only by a difference in the front-joint. There is here no concentration of vitality in special parts of the structure, as in the higher animals, but the nervous force is scattered through the whole body,--every ring having, on its lower side, either two nervous swellings, one on the right, the other on the left side, connected by nervous threads with those that precede and those that follow them, or these swellings being united in the median line. It is this equal distribution of nervous force through the whole system that gives to these animals such an extraordinary power of repairing any injured part, so that, if cut in two, the front part may even reconstruct a tail for itself, while the hind part produces a new head, and both continue to live as distinct animals. This facility of self-repair, after a separation of the parts, which is even a normal mode of multiplication in some of them, does not indicate, as may at first appear, a greater intensity of vital energy, but, on the contrary, arises from an absence of any one nervous centre such as exists in all the higher animals, and is the key to their whole organization. A serious injury to the brain of a Vertebrate destroys vitality at once, for it holds the very essence of its life; whereas in many of the lower animals any part of the body may be destroyed without injury to the rest. The digestive cavity in the Worms runs the whole length of the body; and the respiratory organs, wherever they are specialized, appear as little vesicles or gill-like appendages either along the back or below the sides, connected with the locomotive appendages. This class includes animals of various degrees of complication of structure, from those with highly developed organizations to the lowest Worms that float like long threads in the water and hardly seem to be animals. Yet even these creatures, so low in the scale of life, are not devoid of some instincts, however dim, of feeling and affection. I remember a case in point that excited my own wonder at the time, and may not be uninteresting to my readers. A gentleman from Detroit had had the kindness to send me one of those long thread-like Worms (_Gordius_) found often in brooks an
PREV.   NEXT  
|<   118   119   120   121   122   123   124   125   126   127   128   129   130   131   132   133   134   135   136   137   138   139   140   141   142  
143   144   145   146   147   148   149   150   151   152   153   154   155   156   157   158   159   160   161   162   163   164   165   166   167   >>   >|  



Top keywords:
animals
 

nervous

 

structure

 

appendages

 

swellings

 
connected
 
injury
 

threads

 
vitality
 

length


higher

 

complication

 
typical
 

vesicles

 
presents
 

degrees

 
specialized
 
locomotive
 

includes

 

branch


respiratory

 

essence

 

uniform

 

destroys

 

destroyed

 

highly

 

organs

 

cavity

 

digestive

 

uninteresting


readers

 
gentleman
 

excited

 

classes

 

Detroit

 
brooks
 

Gordius

 
thread
 

kindness

 
creatures

organizations
 

lowest

 
Vertebrate
 
affection
 

remember

 

feeling

 
devoid
 

instincts

 
developed
 

difference