32 CLASSIFICATION AND CREATION 



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 con- 

 trary, 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 de- 

 stroys 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 

 appen dages either along the back or below the 

 sides, connected with the locomotive appen 

 dages. 



This class includes animals of various degrees— — 

 of complication of structure, from those with 

 highly developed organizations to the Worms 

 that float in fresh water like long hairs and 

 hardly seem to be animals, and to those still 

 lower representatives of the class that live in the 

 cavities of other animals. Yet even creatures 

 so low in the scale of life as the Gordius, that 

 long thread-like Worm found often in brooks 

 and called Horsehair by the common people, are 

 not devoid of some instincts, however dim, of 

 feeling and affection. I remember a case in 



