THE EARTHWORM. 15 



have made but brief allusion to them here, and will 

 now conclude this letter with a few words in regard 

 to the nervous system of the Earthworm. 



This is framed in conformity with the general shape 

 of its body, and is as simple as the organs already 

 described. It consists, first, of a minute roimdish 

 mass of nervous substance, termed the "cephalic 

 ganglion," which corresponds to our brain, and is 

 composed of two smaller masses, of equal size, fused 

 together. The " cephalic ganglion" is situated in the 

 head, just above the throat (if the commencement of 

 the long alimentary canal may be so termed), and 

 from each of its hemispheres there proceeds a nervous 

 chord, one of which passes downwards and backwards 

 on either side of the throat, and the two again amal- 

 gamate under the alimentary canal. Hence they are 

 continued, as a single nervous stem, along the whole 

 ventral portion of the body, close to the external sur- 

 face, giving out in each ring a number of branches of 

 remarkable delicacy, which encircle the body, passing 

 round to the creature's back, and imparting sensi- 

 bility to every portion of its frame. (See ! what excru- 

 ciating pain the angler inflicts upon the poor worm 

 when he impales it upon his barbed hook, which he 

 passes from one end of its body to the other !) 



Even in this apparently uninteresting part of the 

 animal's structure a noteworthy trait presents itself, 

 namely, that whUst in our bodies, and those of the 

 vertebrate animals generally, the spinal chord passes 

 along the back ; in the worm, and, let us add, in aU 



