LUMBRICUS 



149 



the extreme posterior end of the animal. If we examine these 

 structures with a lens we shall be able to see that the supra- 

 pharyngeal ganglion gives off small nerves to the sensitive pro- 

 stomium, and that the ventral nerve-cord swells out between each 

 pair of septa, that is, in each segment, into a thicker portion or 

 ganglion which gives off both dorsally and ventrally and on each 

 side three pairs of nerves to the surrounding parts. 



The nervous system of an earthworm thus consists of two 

 supra-pharyngeal ganglia situated in the third segment, a pair 



FIQ. 62. Diagram of the anterior end of Lumbricus herculeus to show the 

 arrangement of the nervous system. After Hesse. 



i, n, m, iv. The first, second, third, and fourth segments. 



1. The prostomium. 2. The cerebral ganglia. 3. The circumoral com- 

 missure. 4. The first ventral ganglion. 5. The mouth. 6. The 

 pharynx. 7. The dorsal and ventral pair of chaetae. 8. The tactile 

 nerves to the prostomium. 9. The anterior, middle and posterior 



dorsal nerves. 10. The anterior, middle and posterior ventral nerves. 



of connecting cords called commissures which form a ring round 

 the pharynx, and a ventral cord which swells out into a ganglion 

 in every segment behind the third. The ring round the mouth 

 and the solid nature of the nervous system are features common 

 to nearly all the Invertebrata, and in those which have a bilateral 

 symmetry and are segmented there are supra-pharyngeal ganglia 

 and a ventral nerve-cord bearing segmen tally repeated ganglia. 



The nervous system is one of the most important organs of the 

 body. It governs and controls the action of every tissue and cell. 

 It receives and registers impressions from the outside world and 

 co-ordinates the movements and activities of every part of the body. 



