36 THE ANIMALS AND MAN 



Make a drawing of the nervous system. 



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

 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 peri- 

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

 into the arteries once more. This sort of a circulatory sys- 

 tem 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 closed system, i.e., arteries leading into capillaries 

 which in turn lead into veins, in no case allowing the blood 

 to pass freely through the spaces of the body. 



For a detailed accoimt of the life and structure of the 

 crayfish see Huxley's "The Crayfish: an Introduction to 

 the Study of Zoology." 



