768 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the brain of vertebrates. Such a comparison is warranted by the fact 

 that when nerves of special sense are present they invariably originate 

 from this collection of nerve-cells. Where a distinct cephalic gan- 

 glion is present it is situated above the oesophagus, while all the other 

 portions of the ganglionic chain lie on the ventral side of the body, the 

 commencement of the chain being connected with the cephalic ganglion 



Fig. 323.— Nervous System op an Articulate. (Perrier.) 



by a collection of circular fibres around the gullet and spoken of as the 

 oesophageal collar. 



The number of ganglia in the articulata is very variable ; there may 

 be twelve or fifteen pairs or but three. The larger the number of 

 ganglia, the greater their tendency to fusion along the middle line. 



In all invertebrates the nervous system is composed of such a series 

 of separate and distinct ganglia. 



