INTERNAL STRUCTURE OF EARTHWORMS 181 
certain names for each distinct region, as follows: the 
mouth cavity; the pharynx (far'inks), with its thick muscular 
walls; the esophagus (e-sof'a-gus), thin-walled and small; 
the crop , a wide pouch ; the gizzard, where food is ground ; 
and the stomach-intestine, a large, thin-walled tract extending 
through the last two thirds of the length of the worm. 
The earthworm has an easily recognized nervous system 
which is found beneath the digestive tube. It consists of a 
continuous, minute, white thread with slight swellings in 
each segment. From these swellings, which are called 
Drawn from the side. (Esophagus also spelled oesophagus.) 
ganglia (gan'gli-a: Greek, ganglion, swelling or tumor), 
short branches extend to the digestive tube and other organs. 
These branches are known as nerves. Toward the front end 
the nerve-thread parts and becomes double. Each part 
passes around the front end of the pharynx and enlarges to 
form two ganglia, the largest found in the earthworm. More 
nerves grow from these two large ganglia than from any 
of the others and so the term “ brain ” is given to these two 
ganglia found in the dorsal surface of the pharynx (Figure 
167). 
The nervous system of the earthworm is clearly defined and 
occupies a central position, being found in the middle of 
the body. Although they have no eyes or ears, earthworms 
