106 NERVOUS SYSTEM OF ARTHROPODS. 



eight abdominal ganglia, which are always much smaller 

 than the thoracic, also continue to have a separate exist- 

 ence among some 6f the less developed types of Insects 

 (fig. 43) though it. is more frequent for some, or even all, 

 of them to disappear (figs. 44, 45). 



The ' stomato-gastric ' system of nerves attains a con- 

 siderable degree of complexity in these animals. In front 

 there is a median ganglion (fig. 42) lying below and often 

 anterior to the brain. This oral ganglion is a swelling 

 situated on the great median (afferent) visceral nerve, at 

 the spot where it bifurcates in order to proceed to each 

 half of the brain. It receives branches from the mouth 

 and adjacent parts. The main nerve, or else the ganglion, 

 is also connected with other branches (c), proceeding from 

 one or two pairs of lateral ganglia situated close to the 

 cesophageal cords, and often in structural relation with 

 them. This visceral system of nerves receives branches 

 from the stomach, the intestines, and other internal 

 organs. 



In Insects, moreover, we meet with another semi- 

 independent set of visceral nerves, connected with a chain 

 of minute ganglia lying upon the great ventral ganglionated 

 cord, and united thereto by means of minute nerve fila- 

 ments. The nerves (fig. 41, o, o, o) in connection with this 

 chain of minute ganglia are received from and distributed 

 to the all-pervading respiratory organs (air tubes) of the 

 Insect. They are known to anatomists, on account of the 

 disposition of their main branches, as ' nervi transversi,' 

 and are much more highly developed in these animals 

 than are anything corresponding to them amongst other 

 Arthropods. 



