62 CLASSIFICATION AND CREATION. 
even a normal mode of multiplication in some of 
them, does not indic&te, 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 
appendages 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 
