XV111 INTRODUCTORY REMARKS. 



VERTEBRATA. Having a single nervous centre, 

 or axis, running through the body, enclosed in a 

 bony case. 



MOLLUSCA. Nervous system consisting of several 

 detached masses, irregularly scattered about the 

 body ; no bony skeleton. 



ARTICULATA. Nervous system consisting of two 

 parallel nervous cords, running through the body, 

 thickened at intervals into knots or ganglions. 



RADIATA. No distinct nervous system ; organs 

 of sense and motion arranged in a circular form, like 

 rays around a centre. 



