330 METAZOAN PHYLA 



brates, however, is generally found a pair of lungs (Fig. 228), together 

 with the windpipe, or trachea, and a voice box, or larynx. The lungs 

 originate as outpocketings from the ventral side of the pharynx and are 

 lined, as is the pharynx, with entodermal epithelium, 



350. Circulatory System. — The blood in vertebrates consists of a 

 fluid plasma in which float white corpuscles, red corpuscles, and blood 

 platelets. The red corpuscles contain hemoglobin, which unites with 

 oxygen to form oxyhemoglobin. In this form the blood carries a much 

 greater amount of oxygen than could be carried if it were only in solution 

 (Sec. 272). The carbon dioxide is carried in the plasma in combination 

 as sodium carbonate. The white corpuscles, or leucocytes, are ameboid 

 in character and are able to ingest and digest foreign particles, including 

 unicellular organisms. These corpuscles escape from the blood through 

 the walls of the capillaries and wander about in the looser tissues. They 

 serve to rid the body of deleterious substances or may help to protect it 

 from disease-producing germs. The blood platelets produce a substance 

 active in bringing about the coagulation of the blood. It is believed that 

 they are formed from white blood corpuscles. The fluid part of the blood, 

 or plasma, contains in solution proteins which may be caused to coagu- 

 late, or clot. Clotting occurs whenever an opening is produced in the wall 

 of a blood vessel and the clot blocks the opening, preventing excessive 

 hemorrhage. When the clot is removed from the plasma, the fluid 

 remaining is termed serum. 



The circulatory system of vertebrates includes a central organ — the 

 heart — which receives the blood returned from the body or, in certain 

 cases, also from the lungs and sends it out again either over the body or 

 to the lungs. In addition there is a closed system of vessels, including 

 arteries, capillaries, and veins. It is through the walls of the capillaries 

 that the interchange of gases and of food and waste takes place. 



The circulatory system also includes the spleen and the lymphatics. 

 The spleen (Fig. 228) is an organ in which old and useless red blood 

 corpuscles are broken down and the products passed into the blood. 

 The lymphatic vessels serve as an accessory return circulation, picking 

 up the plasma which has leaked from the blood vessels all over the body, 

 together with white corpuscles which may be added to it, and pouring 

 both back into the veins a short distance from the heart. This fluid 

 containing plasma and white corpuscles is called lymph. Into the lym- 

 phatic system are also absorbed the products of fat digestion. At inter- 

 vals along the lymphatic vessels are masses of tissue known as lymph 

 nodes, or lymph glands, in which are formed the white blood corpus- 

 cles and in which foreign particles and infectious organisms are removed 

 from the lymph before it is added to the blood. 



351. Excretory System. — The excretory system consists of the 

 kidneys, the ducts leading from them, and in some cases a bladder. 



