338 PHYSIOLOGY OF THE INVERTEBRATA. 



The difference in the composition of the nervous matter of 

 the two animals, may possibly account for the difference 

 in the rates of transmission as observed by Fredericq and 

 Vandevelde. 



The Polyzoa. 



The nervous system of these animals is very simple, and 

 consists of a ganglion, situated between the mouth and the 

 anus (see Fig. 1 7), which gives off many nerve-fibres to the 

 tentacula and the alimentary canal. 



The Brachiopoda. 



The nervous system of the Clistcntcrata consists of a ganglion 

 on the ventral side of the oral aperture. From this ganglion 

 proceeds a commissure, which surrounds the oesophagus, and 

 bears two small ganglia. " The latter probably answer to the 

 cerebral, the former to the podal, ganglia of the Lamdlihran- 

 chiata. Immediately behind the pedal mass, from which two 

 large nerves to the dorsal or anterior lobe of the mantle are 

 given off, are two elongated ganglia, connected by a commis- 

 sure of their own, which possibly correspond with the parieto- 

 splanchnic ganglia of the higher Molluscs. The nerves to the 

 ventral lobe of the mantle, and those to the peduncle arise 

 from these ganglia." 



The nervous system of the Trctcnterata has not been so 

 thoroughly worked out as that of the Clistcntcrata. ; but in 

 Lingula, Sir Richard Owen, F.K.S.,has shown that the visceral 

 nerves are more developed than those of Terchratiila, which 

 belongs to the latter order. '• Filaments to the muscles are 

 also more distinct : a pair, which come off from the sub- 

 oesophageal ganglion, diverge as they pass backwards along 

 the visceral chamber, then converge to their insertion in the 

 anterior muscles ; a second pair, also from the sub-oesophageal 

 ganglia, run more parallel as they pass along the ventral 

 aspect of the anterior muscles to go to the posterior muscles. 



