TUBIFEX. 333 



THE NERVOUS SYSTEM. 



Tlie nervous system of Tubifex rivulorum is formed 

 upon the same plan as that of all Oligochaeta. It consists 

 of cerebral ganglia united to a ventral chain of ganglia 

 or nerve cord by peripharyngeal connectives. As in most 

 other Oligochaeta the whole nervous system lies com- 

 pletely in the body cavity. It is possible to examine the 

 arrangement of the ganglionated chain and peripheral 

 nerves in the living worm, especially after the addition 

 of ether to the water in which the worm is lying. The 

 ether increases the transparency of the body wall so that 

 the internal organs can be seen more easily, and in some 

 cases the alimentary canal becomes so contorted that the 

 ventral nerve cord is left quite freely exposed for a 

 considerable distance. This is a very useful method of 

 investigation, as it enables one to compare the form and 

 size of the ganglia, and the proportion of connective to 

 ganglion in the segment in different parts of the body, 

 and these proportions vary a good deal. It is, however, of 

 very little use for a detailed examination of the brain. 

 In the living worm it is usually very difficult to define 

 the outlines of the brain, even after the addition of 

 ether. It has happened, however, very occasionally, that 

 I have been able to see the brain quite clearly by this 

 method. The difficulty which is usually experienced in 

 examining the brain in the living condition is due partly 

 to the greater transparency of the brain compared with 

 that of the body wall, and partly to the contractions of 

 the pharynx over which the brain is situated. The great 

 (1 ia\\ back to the use of ether is that the blood vessels in 

 the anterior region of the body heeoine somewhat 

 distended and quite full of blood. This, of course, only 

 serves to obscure the I) rain more. 



