82 



THE WORM GROUP 



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 

 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 81). 

 The organs of the earthworm are supplied with blood 

 which is carried in a large dorsal blood vessel, a ventral 

 blood vessel, and numerous branches. The blood is 

 pumped by the contracting of the dorsal vessel and by the 

 five pairs of tubes which pass from the dorsal to the 

 ventral vessel around the esophagus. These five tubes 

 are named aortic (a-6r'tlk) arches. 



Figure 81. — Earth- 

 worm. 



Front end of nervous 

 system. 



LABORATORY STUDY OF INTERNAL STRUCTURE 



Work out the internal structure of the earthworm. In dissecting, cut 

 the skin along the dorsal surface, being careful to cut the many 

 membranes that hold the digestive tube in place. Work out the size and 

 position of the mouth cavity, pharynx, esophagus, crop, gizzard, and 

 stomach-intestine. The white reproductive organs are located beside the 

 esophagus. Locate the "brain," the ventral chain of ganglia. The 

 dorsal blood vessels and aortic arches should be located. Make a sketch 

 locating the organs in their respective segments. 



