THE CRAYFISH 25 



passing upward on each side of the oesophagus, where they 

 again meet to form the supra-oesopJiageal ganglion or 

 brain. Where do the nerves run which rise from the 

 brain ? What is the difference between the position of 

 the central nervous system in the crayfish and in the toad ? 



Make a drawing of the nervous system. 



Just beneath the nerve-cord note a blood-vessel ex- 

 tending the length of the body. This is the sternal 

 artery, which arises from the posterior end of the heart 

 and passes ventrally at one side of the alimentary canal 

 and between the nerve-cords. Here the sternal artery 

 divides into an anterior and a posterior branch, from 

 which lesser branches are given off to each one of the 

 appendages. The various arteries running to all parts of 

 the body finally pour out the blood into the body-cavity, 

 where it flows freely in the spaces among the various tissues 

 and organs. After the blood has bathed the body tissues 

 it flows to the gills on either side, passing up the outer 

 side of the gill through delicate thin-walled vessels, where 

 it is oxygenated as has already been described. From 

 the gills the purified blood flows back on the inner side 

 through a large chamber, sinus, into the pericardium, 

 through the ostia of the heart, whence it is driven into the 

 arteries once more. This sort of a circulatory system in 

 which the blood in places is not enclosed in a definite 

 vessel is known as an open system. In the toad we find 

 the blood in a dosed system, i.e., arteries leading into 

 capillaries which in turn lead into veins, in no case allow- 

 ing the blood to pass freely through the spaces of the 

 body. 



