412 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



off from the supracesophageal ganglia, we find that in 

 the earthworm, for example, several nerves are given 

 off from the cesophageal commissures, and that each 

 successive ganglion gives off two nerves on either side, 

 while one nerve on either side is given off by the cords 

 which connect the ganglia with one another. When 

 we come to a more differentiated form, such as the cray- 

 fish, we find that no nerves are given off from the 

 commissures, but that three pairs of nerves are sent off 

 from each of the ganglia that belong to one segment 

 only, while, when two or more ganglia have fused 

 together, a large number of nerves are given off in 

 order to supply more than one segment of the body. 



In addition to the sensory and motor nerves there 

 are others which are particularly related to the diges- 

 tive and circulatory organs ; these are the so-called 

 visceral nerves, and, from a physiological standpoint, 

 if not indeed also from a morphological, they are 

 comparable to the system which, in Man and other 

 vertebrates, is spoken of as the sympathetic system. 

 While in the lower worms these visceral nerves are 

 merely cords given off from the cerebral ganglia, they 

 become more independent in the higher forms, owing 

 to the development of ganglia along their course ; a 

 well-marked ganglion of this kind may be seen on the 

 dorsal surface of the crop of the cockroach. The 

 general arrangement of the " stomato-gastric " system of 

 this animal will serve conveniently as a type, and may 

 be thus described ; from the anterior part of the cere- 

 bral mass a cord arises on either side, which, after 

 passing forwards for a short distance, bends on itself 

 and unites with its fellow in a median ganglion. The 

 single cord given off from this ganglion passes back- 

 wards beneath the brain to another median ganglion ; 

 with this last two lateral ganglia are connected ; the 

 second median ganglion gives off a cord which passes 

 backwards above the digestive tract to a third ganglion 



