CHAPTER V. 



THE NERVOUS SYSTEM OF VERMES. 



Nothing distinctly answering to a Brain is to be found in 

 some other of the lowest animals in which a nervous 

 system exists. It is thus, for instance, with Star-fishe3 

 and the larger Nematoid Entozoa. What most nearly 

 resembles such an organ in Star-fishes, consists of a mere 

 band of nerve fibres,, surrounding the commencement of 

 the oesophagus, and containing a few nerve-cells partly 

 between its fibres and partly in groups slightly removed 

 therefrom. The absence of any distinct ganglia in the 

 neighbourhood of the mouth is doubtless due, in the main, 

 to the form of these animals, and their low type of organi- 

 zation. Each arm or ray contains its own nervous 

 system, so that the ring or band round the mouth seems 

 to be little more than a commissure connecting such 

 otherwise distinct parts of the common system. These 

 Echinodcrms are, however, here only incidentally re- 

 ferred to. 



In the larger parasitic Nematoids the nervous system is 

 more concentrated. The oesophageal ring and imme- 

 diately adjacent parts constitute almost all that is as yet 

 known of their nervous system, but it contains, or is in 

 relation with, a larger number of ganglion-cells than 

 the similar part in Star- fishes. Thus, in addition to the 

 cells intermixed with the fibres of the ring itself, there are 

 five or six groups adjacent to and in connection with it, 



