THE EARTHWORM 201 



than in the contrary direction. If the pure water in the ex- 

 periment were replaced by a colloid-solution, then the pas- 

 sage of liquid would practically all be from the salt-solution 

 into the colloid-solution. 



It is well known that a crystalloid, like salt or sugar, if 

 placed in a vessel of water, will soon diffuse through the water. 

 In doing so it exerts a certain amount of pressure. This is 

 called osmotic pressure. If a permeable membrane be inter- 

 posed between a solution of crystalloids and one of colloids, 

 both substances will exert some osmotic pressure, but by 

 far the greater amount is exerted by the crystalloid, because 

 of its greater facility in diffusing through water. Applying 

 the principle of osmosis to absorption in the intestine cf the 

 earthworm, we. have crystalloids in the cavity of the intes- 

 tine, a permeable though not porous membrane (the intestine- 

 wall), and colloids in the blood and body-cavity fluid. The 

 digested food diffuses through the intestine-wall because of 

 the pressure it exerts in seeking to mix with more water. 

 Food on passing to the blood is changed by the cells of 

 the intestine-wall from the crystalloid condition to a col- 

 loid, and is henceforth incapable of passing back through the 

 membrane. 



Circulation. Whenever an animal body is too large or com- 

 plicated for the food to reach all the cells directly, we find 

 a circulatory system developed. The circulatory system of 

 the earthworm is complete, and rather complex. The dor- 

 sal blood-vessel (Fig. 100, 17) extends the length of the animal, 

 along the middle, between the body- wall and the alimentary 

 canal. The blood flows toward the anterior region in some- 

 what regular " pulses," carried along by waves of muscular 

 contraction in the wall of the blood-vessel itself. Between 

 the pharynx and the crop are situated five pairs of "hearts" 

 (Fig. 100, 19, 20). These short tubes receive most of the 

 blood that comes forward, take up the waves of contraction- 



