METABOLISM AND TRANSPORT IN ANIMALS 



525 



testine and glucose soon appears in the 

 blood stream. Fatty acids form a complex 

 with certain bile salts. This complex is solu- 

 ble and passes by diffusion into epithelial 

 cells. There it is converted into a lipid, and 

 much of it passes into the lacteals in the in- 

 testinal villi, and is carried to the thoracic 

 lymph duct which empties into a vein in the 

 neck. Glycerol is absorbed by diffusion and 

 enters the blood stream. Absorption of the 

 fat-soluble vitamins A, D, and K develop in 

 the presence of bile salts. Without bile, a 

 deficiency of these vitamins may occur. 

 The 5 million villi increase the internal sur- 

 face of the small intestine enormously, thus 

 rendering this part of the human digestive 

 tract particularly favorable for absorption. 



Diffusion and osmosis (Fig. 8) take part 

 in absorption, but, as noted above, other 

 cellular activities are involved. Small mole- 

 cules such as those of amino acids pass 

 through cell membranes by way of minute 

 holes too small for large molecules, such as 

 those of cane sugar and of proteins. This 

 does not mean, however, that passage always 

 occurs from the digestive cavity, where the 

 concentration of digested substances is high, 

 into the cells, where the concentration is 

 lower. As a matter of fact, the absorptive 

 cells seem to allow the passage of certain 

 substances and reject the passage of others 

 regardless of their concentration in the diges- 

 tive cavity or within the cells. How this is 

 brought about is still unexplained, but it 

 must be regulated by cell activities which re- 

 quire expenditure of cellular energy; if cells 

 are killed, substances should pass through 

 their cell membranes according to the phy- 

 sicochemical laws of diffusion and osmosis, 

 but actually they do not. 



CIRCULATORY SYSTEMS AND 

 INTERNAL TRANSPORTATION 



Circulation in invertebrates 



Having captured, ingested, digested, and 

 absorbed food, the animal must now trans- 



port it to all parts of the body. In the Pro- 

 tozoa, this is accomplished by the stream- 

 ing of the cytoplasm which carries the food 

 vacuoles from one place to another. In the 

 sponges, digested food is passed from cell 

 to cell and carried from place to place by 

 amoeboid wandering cells. In hydras and 

 other coelenterates, food that is digested in 

 the gastrovascular cavity comes in contact 

 with the inner body wall throughout this 

 cavity, is absorbed by certain gastrodermal 

 cells, and part of it is passed on to other 

 gastrodermal cells and to epidermal cells. 

 A similar method of distribution occurs in 

 the much-branched intestine of flatworms. 



In the earthworm, we encounter a com- 

 plicated system of tubes, the circulatory sys- 

 tem, which carries digested food to all parts 

 of the body. A similar system is present in 

 most types of higher animals. In the crusta- 

 ceans (crayfish), large body spaces (sinuses) 

 are present in the midst of the tissues into 

 which blood from the arteries accumulates 

 and from which it passes into the heart after 

 flowing through the gills. In the insects 

 (grasshopper), the body cavity is a hemocoel 

 filled with blood which bathes the tissues. In 

 the echinoderms (starfish), the circulatory 

 system is rather poorly developed; digested 

 food passes into the coelom, where cilia 

 keep the coelomic fluid in motion, thus dis- 

 tributing it. 



Circulation in vertebrates 



Blood and lymph 



In the vertebrates the circulatory system 

 (Fig. 386) consists of a heart, arteries, veins, 

 and lymph vessels. Human blood is bright 

 red in arteries and dark red in veins. It 

 makes up about Yi^ of the body weight. It 

 consists in volume of about Vi liquid plasma 

 and Vz formed elements (Fig. 382). The lat- 

 ter are (I) red corpuscles (erythrocytes), 

 (2) blood platelets, and (3) white cells 

 (leukocytes). The leukocytes are of 5 types: 

 lymphocytes, monocytes, neutrophils, eosin- 

 ophils, and basophils. 



