342 



ZOOLOGY. 



*/-*re: 



COAO 



Fig. 177. CENTRAL NERVOUS 



SYSTEM OF EARTHWORM, 



ANTERIOR END. 



(After Howes.) 



between the buccal cavity and the pharynx, are known as 

 the cerebral or supra-cesophageal * ganglia. They are 

 connected to the ventral nerve-cord by a pair of circum- 



cesophageal * commissures. The 

 nerve-cord is very slightly thicker 

 towards the middle of eachsomite, 

 and this has led to its being in- 

 accurately described as a " chain 

 of ganglia " : but the thickenings 

 are not ganglia, because nerve- 

 cells are not confined to them. 

 The cord is just as much a con- 

 tinuous cord as is the spinal cord 

 of Vertebrates. It differs from 

 the latter not only in its ventral 

 position, but also in structure. 

 It has no central canal ; the 

 nerve-cells are aggregated to the 

 ventral side, the longitudinal 

 fibres to the dorsal side (cf. grey 



and white matter). At the extreme dorsal side there are 

 three giant fitoes, resembling nerve-fibres in structure but 

 of much greater diameter : they may be supporting 

 structures only. 



Nerves are given off metamerically three pairs in each 

 segment but they have no separate roots for their sensory 

 and motor fibres. The sensory fibres differ from those of 

 all the nerves of Vertebrates except the olfactory, by being 

 outgrowths of cells in the sensory epithelium (in this case the 

 epidermis) : they resemble the sensory fibres of a Vertebrate's 

 spinal nerves, however, in their behaviour on reaching the 

 cord, as they divide into an ascending and descending 

 branch, each of which ends in arborizations around a nerve- 

 cell. The motor-fibres originate from nerve-cells in the 

 cord, as in Vertebrata. 



There are no specialized sense-organs eyes, ears, or 

 olfactory organs. Scattered through the epidermis, but 



* These terms (as applied to the earthworm) are inconsistent 

 with the sense in which the term "oesophagus" is used, as they lie 

 in front of the " pharynx." 



