432 THE NEKVOUS SYSTEM IN THE LOWEK AJSTIMALS 



ring of ganglia is actually inclosed in a cartilage box, thus 

 giving it a still closer resemblance to a brain. 



Worms.— In the worms we find two main ganglia 

 usually located in the head end and united by a com- 

 missure. In the simpler wonn forms these ganglia are 

 the only ones in the body, and fibers from them supply all 

 parts of the body. 



In the earthworm these two ganglia are represented 

 by a single two-lobed mass located just above the esopha- 

 gus and called on this accoimt the supra-esophageal 



v.n. 



Fig. 201 — Nervous system of earthworm; (/, ganglion; v^n, ventral nerve cord. 



ganglion. From this compound mass or " brain " a 

 ring of cord encircling the esophagus unites it with a 

 ganglion mass under the esophagus called the sub-esopha- 

 geal ganglion. Finally, a double cord extends from this 

 latter ganglion along the ventral side of the body. This 

 cord shows a series of small ganglia at intervals corres- 

 ponding to segments. Each of these small ganglia is the 

 source of motor and sensory fibers which supply that 

 particular segment, and these segment ganglia are the seat 

 of the reflex actions. It is not difficult, then, to compare 

 such a system with the one in man. The supra-esophageal 

 ganglion may be compared to the brain, the ventral cord 

 to the spinal cord, and the smaller ganglia to the centers 

 of reflex actions and sources of spinal nerves in the cord. 



