138 THE BIO LOOT OF AN ANIMAL. 



ganglia in various parts of the body. Of these the most important are the 

 pharynyeal ganglia, 3 to 5 in number which lie on the wall of the pha- 

 rynx on each side just within the pharyngeal collar. They are connected 

 with the latter by fine branches, and send minute nerves out upon the 

 walls of the pharynx. This series of ganglia is often inappropriately called 

 the sympathetic system. 



Peripheral Nervous System. To and from the central system 

 just described run the nerves which constitute the peripheral 

 system. These are as follows : 



1. A pair of nerves running out on either side of each ven- 

 tral ganglion and lost to view among the muscles of the body- 

 wall (l.n, Fig. 80). 



2. A single nerve proceeding from the ventral commissures 

 on each side immediately behind the dissepiment to which it is 

 mainly distributed. 



3. A pair of nerves (Fig. 80), in front of the foremost pair 

 of ventral ganglia. 



i. A pair of nerves from the point where the two ventral 

 cords diverge in front of the next dissepiment to form the pha- 

 ryngeal collar. 



5. A nerve from each half of the collar just beyond its di- 

 vergence from its fellow. (Origin incorrectly shown.) 



6. Two large nerves (cerebral), which run forwards from the 

 cerebral ganglia, break up into many branches, and are distributed 

 to the anterior part of the body. 



The branches of the various nerves ramify through all parts 

 of the body. Many of them notably those of the cerebral gan- 

 glia end in the skin, and conduct impulses inwards to the nerve- 

 centres (afferent nerves). Some end in the muscles, and conduct 

 impulses outwards to them from nerve centres (efferent or motor 

 nerves). Others terminate in the walls of the stomach, nephridia, 

 blood-vessels, etc., and among these are probably both afferent 

 and efferent nerves. It is highly probable, from analogy with 

 higher animals, that the afferent and efferent nerve-fibres are al- 

 ways distinct i.e., that the inward and outward impulses never 

 travel through the same nerve-fibre. 



Nervous Action. Nerves are normally thrown into action by 

 certain changes or stimuli affecting their extremities. Thus a 

 touch or pressure upon the skin acts as a stimulus to the nerves 



