178 ESSENTIALS OF ZOOLOGY ■ 9 



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 substances, 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 will be 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, i.e., the vessels carry the blood through its entire 

 circulation without being interrupted by extensive sinuses, 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 mol- 

 lusks and crustaceans there is a similar respiratory pigment carried 

 in the plasma, which is called hemocyanin. Instead of iron, copper 

 is the principal constituent of this pigment. Vertebrate blood is 

 largely water carrying dissolved materials and suspended corpuscles. 

 The fluid part is known as plasma. The amount of blood in a mammal 

 is approximately one-tAventieth of the body weight, or in the average 

 man a little more than a gallon. The plasma contains enough inor- 

 ganic 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 un- 

 equally 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 skeletal muscles require another fourth; 

 and the remaining fourth is distributed through all of the other 

 organs. Human blood contains normally about 5,000,000 red cor- 



