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EVOLUTION OF ANIMALS 



Part V 



all of absorption occur. Its enzymes are cellulase that acts on the cellulose of 

 plant tissues, amylase on carbohydrates, pepsin and trypsin on proteins and 

 lipase on fats. Absorption of food takes place through the ciliated epithelium 

 that lines the intestine. The absorptive surface is increased by the bulging out 

 of the intestinal wall in separate segments and by an infolding of its dorsal 

 wall — the typhlosole. The digested food is here either taken up by the blood 

 into the numerous capillaries embedded in the intestinal wall, or directly into 

 the coelomic fluid. A greenish layer of chloragog cells, important in excretion, 

 covers the blood vessels and intestine. 



Other Metabolic Processes. Circulation, The body fluids are the watery 

 coelomic fluid, the tissue fluid in direct contact with the cells, and the red 

 blood. The red blood consists of the red plasma with the respiratory pigment 

 hemoglobin in solution in it and the colorless ameboid cells. The blood circu- 



5 pairs of hearts 



Body wall 



Dorsal 



Ventral v. 



Nerve cord 



Sub neurol v. 



Fig. 28.8. Main vessels in the forepart of the earthworm. Blood is forced 

 through the large dorsal vessel by waves of contractions that begin at its posterior 

 end and pass forward, backward flow being prevented by valves. Along the way 

 it is distributed to side branches, largely to the hearts which connect with the main 

 ventral vessel. Small branches from this carry it to the kidneys (nephridia) and 

 body wall. By way of various vessels it is finally returned to the posterior end of 

 the dorsal vessel and carried forward again. 



lates through a system of tubes that branch to all parts of the body. The dorsal 

 vessel lying on the digestive tube is the ffiain collecting vessel (Figs. 28.8, 

 28.9). By its rhythmic contractions, this vessel and the five pairs of hearts 

 determine the direction of the flow of blood through them and backward 

 through the long ventral vessel that lies directly beneath the digestive tube. In 

 almost every segment blood flows out of the ventral vessel to the dorsal one 

 by way of the capillaries in the body wall, digestive tube, and kidneys. The 

 subneural vessel also carries blood backward, supplies the nerve cord, and has 

 branches that connect with the dorsal vessel. Valves in the hearts prevent back 

 flow as they do in the veins of higher animals. 



There are special distributions of blood vessels to very vital structures: to 

 the skin in which respiratory gases are exchanged; to the digestive tube with 

 its food supply; to the kidneys concerned with water balance and excretion; 

 to the muscles and to the nerves that depend upon abundant oxygen. 



