8 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



numerous and give the red color to the blood. They contain 

 a substance called hemoglobin, which takes up the oxygen in 

 the respiratory organs and carries it to the other tissues of the 

 body. The white corpuscles, or leucocytes, are amoeboid in 

 character, and are able to change their shape and move about 

 independently. It is their duty to take up and destroy any 

 small foreign objects such as bacteria, parasitic germs, bits of 

 broken-down tissue, or other particles that should be elimi- 

 nated. In this way they prevent the undue multiplication of 

 many disease germs that might prove most serious if they 

 attained to considerable numbers. The spindle-cells are cor- 

 puscles which may later develop into red corpuscles. Most 

 of the blood corpuscles are developed in the marrow of the 

 bones, although many of the white corpuscles are formed in 

 the spleen, which is a reddish, oval body lying above the ante- 

 rior end of the cloaca. The blood is contained in a system of 

 veins and arteries with a central pumping station, the heart, 

 which drives the blood out through the blood-vessels to all 

 parts of the body. With the aid of a microscope the blood may 

 plainly be seen circulating through the membrane between 

 a live frog's toes. Besides the red blood in the closed circu- 

 lation, there is a colorless lymph containing white corpuscles 

 occurring in many lymph spaces in various parts of the body. 

 The lymph is derived from the plasma of the blood and ulti- 

 mately flows back into the veins. The lymph spaces connect 

 with each other, and the large lymph hearts in the dorsal part 

 of the body cavity, by their pulsations, drive the blood into 

 two of the veins in the region of the heart. 



The pear-shaped heart is enclosed in a delicate semi-trans- 

 parent sac, the pericardium. It is made up of the conical 

 muscular ventricle and the thinner-walled right and left auricles. 

 When the ventricle contracts, the blood is driven out through 

 the thick-walled truncus arteriosus, which soon divides. Each 

 of the divisions gives off three branches, the carotid arteries, 

 which supply the head, the systemic arteries, which pass around 

 the alimentary canal and unite above forming the dorsal aorta, 

 and the pulmonary arteries, which carry blood to the lungs and 

 the skin. The systemic arteries and the aorta give off branches 



