THE EARTHWORM AND ANNELIDS IN GENERAL 171 



probably present in the digestive fluids of the earthworm. The 

 proteids are changed into peptones, the carbohydrates into a 

 sugar compound, and the fats are divided into glycerin and fatty 

 acids. 



ABSORPTION. The food is now ready for absorption. This is 

 accomplished through the wall of the intestine by a process 

 known as osmosis, assisted by an ameboid activity of some of 

 the epithelial cells. Osmosis is the passage of a liquid through 

 a membrane. 



CIRCULATION. Upon reaching the blood, the absorbed food is 

 carried to various parts of the body by circulation, the details of 

 which are described in another place (pp. 174-175). Absorbed 

 food also makes its way into the ccelomic cavity and is carried 

 directly to those tissues bathed by the ccelomic fluid. In one- 

 celled animals, and in such Metazoons as Hydra, Planaria, and 

 Ascaris no circulatory system is necessary, since the food either is 

 digested within the cells or comes into direct contact with them; 

 but in large, complex animals a special system of organs must be 

 provided to enable the proper distribution of nutriment. 



ASSIMILATION, as in the types already described, is the addition 

 of new particles among the preexisting particles of protoplasm. 



Vascular System. The blood of the earthworm is contained 

 in a complicated system of tubes which ramify to all parts of the 

 body. A number of these tubes are large and centrally located; 

 these give off branches which likewise branch, finally ending in 

 exceedingly thin tubules, the capillaries. The functions of this 

 system of tubes are to carry nourishment from the alimentary 

 canal to all parts of the body, to transport waste products, and to 

 convey the blood to a point near the surface of the body where 

 oxygen may be obtained and supplied to the tissues. 



BLOOD. The blood of the earthworm consists of a plasma in 

 which are suspended a great number of colorless cells, called 

 corpuscles. Its red color is due to a pigment termed hcemoglobin, 

 which is dissolved in the plasma. In vertebrates the haemoglobin 

 is located in the blood corpuscles. 



