90 THE NERVOUS SYSTEM OF VERMES. 



the muscles whose business it is to move its three saw- 

 like jaws, as well as to the muscles of the oral sucker. 

 This lower ganglion is in part analogous to the ' me- 

 dulla oblongata ' of vertebrate animals. It is continuous 

 with the double ventral cord, on which twenty equidistant 

 rhomboidal ganglia are developed. Each of these ganglia 

 gives off two nerves on either side, whose branches are 

 distributed to the parietes and the muscles of adjacent 

 segments. 



In this animal a simple filament is also given off from 

 the posterior part of the supra-ossophageal ganglion, which 

 is distributed along the dorsal aspect of the alimentary 

 canal. It foreshadows an important system of fibres in 

 higher animals, corresponding partly with the pneumo- 

 gastric nerves, and partly with the ' sympathetic system.' 

 As it exists amongst the Invertebrates it is known as the 

 ' stomato-gastric system ' of nerves. In other members 

 of the invertebrate series it frequently takes origin from 

 the commissures connecting the upper and lower oeso- 

 phageal ganglia, rather than from the upper ganglia them- 

 selves. In some of the worms, in which such an ar- 

 rangement exists, the stomato-gastric system is also more 

 complicated. 



In the Earthworm the body is composed of a multitude 

 of ring-like segments, provided with lateral setae which 

 the animal calls into play during its subterranean loco- 

 motions. It possesses no distinct ocelli, and, having 

 regard to its mode of life, this is not surprising. 



The supra-cesophageal ganglia, which together represent 

 the brain of the Earthworm, receive a nerve trunk on each 

 side, composed of fibres corning from the tactile upper lip ; 

 and, as no sensory filaments of a different order are known 

 to be immediately connected therewith, the functions of 

 the brain in this animal must be comparatively simple. 



