394 TEXTBOOK OF ZOOLOGY 



is a fluid similar to plasma which has seeped through the walls of 

 the capillaries in the various organs, and it carries amoeboid white 

 corpuscles. Certain of them are produced in the lymph glands. 

 The spleen is a lymphoid organ in which debilitated red corpuscles 

 are disintegrated and the products placed in the blood. 



Circulation. — Transportation of materials through the protoplasm 

 of a single cell or a single-celled organism and from cell to cell of 

 the metazoan is a fundamental function among living things. In 

 most Protozoa there is no special arrangement for this function, but 

 the necessary exchange and movement of food materials, waste sub- 

 stances, and gases is accomplished by simple diffusion of materials. 

 In a few forms, however, of which Paramecium is an example, there 

 is a definite course of movement by the endoplasm. This is known 

 as cyclosis, and it serves to circulate the food vacuoles. 



In double-walled, simple, saccular forms like hydra there is no pro- 

 vision necessary except an exchange of the water in the gastrovascu- 

 lar cavity. In flatworms, such as planaria, the necessity of increased 

 food distribution is cared for by branching of the gastrovascular 

 cavity into diverticula. In sponges the wandering cells assist in 

 transporting materials. A distinct system of tubelike vessels with 

 contractile parts is developed in the annelid worms, as was studied 

 in the earthworm. Here a closed system of vessels forms a complete 

 circuit to carry a circulating medium to all parts of the body. In 

 this group the fluid is known as hemolymph because it bears no red 

 corpuscles. The hemoglobin is borne in the fluid. The vertebrate 

 system is closed, and the blood is circulated by the action of a single 

 heart. The hemoglobin, an iron compound, is carried in the red blood 

 corpuscles. In molluscs and some crustaceans there is a similar 

 respiratory pigment carried in the plasma, which is called hemo- 

 cyanin. Instead of iron, copper is the principal constituent of this 

 pigment. Vertebrate blood is largely water carrjdng dissolved mate- 

 rials and suspended corpuscles. The fluid part is known as plasma. 

 The amount of blood in a mammal is approximately one-twentieth of 

 the body weight, or in the average man a little more than a gallon. 

 The plasma contains enough inorganic salts to taste slightly salty. 

 Its salt content is about equal to that of sea water. When the body 

 is active, the blood is very unequally distributed. One-fourth is 

 always in the heart, large arteries, veins, and lungs. Another fourth 

 is held in the hepatic portal system, the liver and its sinuses; the 



