174 PHYSIOLOGY 



a well-developed muscular and nervous system. The 

 nervous system is similar to that of segmented worms. 

 It is made of a double ventral chain of ganglia con- 

 nected with a nerve cord that divides at its anterior end, 

 passes around the alimentary canal and unites on the 

 dorsal side in a concentration of ganglia called the brain. 

 Each segment contains a single ganglion which sends off 

 two pairs of nerves to control the muscles and other 

 organs of that segment. When the segments become 

 fused the ganglia often become fused, but it is possible 

 to determine the number of segments involved by count- 

 ing the nerves that are given off. 



Digestive System. — A well-developed alimentary ca- 

 nal open at both ends to the outside extends the length 

 of the body. A large liver furnishes digestive juices. 



Respiratory System. — Special organs are set apart for 

 respiration which take the form of gills in those animals 

 that live in the water and trachea, or air tubes, in those 

 that are surrounded by air. The gills are outgrowths of 

 the body wall covered with a thin, moist membrane 

 and permeated with numerous blood vessels. Oxygen 

 dissolved in the water and carbon dioxide in the blood 

 pass readily through the membrane. Tracheae are tubes 

 which permeate the body and become filled with air. 

 Oxygen can pass from the air in these tubes into the 

 blood or directly into the tissues. 



Excretory Organs. — There are two kinds of excretory 

 organs, both of which take the form of tubes. The neph- 

 ridia, sometimes called the green gland, or shell gland, 

 are in the anterior part of the animal and open to the 

 outside. The Malpighian tubes present in some forms 



