AND PHYSIOLOGY. 



363 



may be called a brain. In 

 Insects " the central parts 

 of the nervous system con- 

 sist of a brain and a ven- 

 tral cord,' 7 as is shown on 

 Fig. 336. In some of the 

 lower tribes of Articulates, 

 as the Helminthes and Rota- 

 toria, the nervous system 

 is feebly and indistinctly de- 

 veloped. 



668. Reflex Actions in Ar- 

 ticulates. — This sub-king- 

 dom is remarkable for its reflex actions. Thus if the head of a 

 centipede be cut off while it is in motion, the body will con- 

 tinue to move by the action of its legs ; and the same will 

 take place, if the body is divided into several segments. The 

 explanation is as follows. "The body is moved forward by 

 the regular and successive action of its legs, as in the natural 

 state : but its movements are always forwards, never back- 

 wards, and are only directed to one side, when the forward 

 movement is checked by an interposed obstacle. Hence, al- 

 though they might seem to indicate consciousness and a guid- 

 ing will, they do not so in reality : for they are carried on as 

 it were mechanically, and show no direction or object, no 

 avoidance of danger. If the body be opposed in its progress 

 by an object of not more than half its height, it mounts over 

 it, and moves directly onwards, as in its natural state : but if 

 the obstacle be equal to its own height, its progress is ar- 

 rested, and the cut extremity of the body remains forced up 

 against the opposing substance, the legs still continuing to 

 moveP 



Fig. 336. 



Nervous System of a Beetle. 



What is the name of the principal ganglia? 668. What is sai:i of the reflex actions 

 of articulate animals ? What are these reflex actions somt times mistaken for* Giva 

 the example. 



