160 BIOLOGY 



into the dorsal vessel, dv, thus bringing the blood back again 

 into the dorsal vessel. There are numerous other small ves- 

 sels, some of which are shown in Figure 77, but the chief ones 

 are those that have been described. 



The blood is forced onward by the contraction of the walls 

 of the dorsal vessel and the hearts, which are provided with 

 valves preventing any back flow when the contractions occur. 

 The course of the blood is rather indefinite and the pure and 

 impure blood are not distinctly separated from each other, as 

 in higher animals. There are no true arteries or veins, and no 

 true hearts. This blood is associated With respiration, and also 

 carries nourishment from the absorbing organs in the intestine 

 to the active tissues, and carries waste products from the active 

 cells to the excreting organs. 



Coelomic or Perivisceral Fluid. The chambers of the body 

 cavity are filled with a fluid called the ccelomic or perivisceral 

 (Gr. peri = around -fLat. viscera = internal organs) fluid, which 

 serves also as a circulatory medium. The food that is absorbed 

 makes its way into the body cavity and is partly absorbed 

 by this fluid. This liquid is forced irregularly backward and 

 forward through the cavity of the body by the motions of 

 the animal, and the nutritious parts of the food which are dis- 

 solved in it are thus directly carried to and fro and brought 

 in contact with the living tissues of the body, that are bathed 

 in this liquid. There is no distinct circulation of this fluid, and 

 it cannot properlybe called a circulatoryfluid. It does, however, 

 have some of the functions of the blood, since it carries to and 

 fro a part of the material absorbed from the digestive tract. 

 It corresponds more closely to the lymph of higher animals. 



Respiration. The earthworm has no distinct respiratory 

 system, but the blood vessels in their circulation in the skin 

 are brought into a very close proximity with the air. Gases are 

 readily exchanged through the thin skin, and respiration is 

 carried on easily without any special respiratory organs except 

 the minute blood vessels that lie beneath the skin. 



