210 



ZOOLOGY 



Diagrammatic cross section of the body of a 

 coelenterate A, and that of a worm B, 



soil. Inside the mouth opening is a part of the food tube called 

 the pharynx. This is very muscular so that it can be extended and 

 withdrawn by the worm. When applied to the surface of any small 

 pebble or leaf, it acts as a suction pump and draws it into the 

 food tube. As the worms take organic matter out of the ground 

 as food, they pass the earth through the body in order to get this 



food. The earth is mixed 

 with fluids poured out from 

 glands in the food tube, and 

 is passed out of the body 

 and deposited on the sur- 

 face of the ground, in the 

 form of little piles of moist 

 earth. These are familiar 

 sights on all lawns; they 

 are called worm casts. 

 Charles Darwin calculated that fifty ythree thousand worms may be 

 found in an acre of ground, and that ten tons of soil might pass 

 through their bodies in a single year to be brought to the surface. 

 Earthworms, in spite of their fondness for some garden vegetables 

 and young roots, do an immense amount of good by breaking up 

 the soil, thus allowing water and oxygen to penetrate to the roots 

 of plants. 



Comparison between Hydra and Worm. — The digestive tract of 

 the worm is an almost straight tube inside of another tube. The 

 latter is divided by partitions which mark the boundary of each 

 segment. The outer cavity is known as the body cavity. In the 

 hydra no distinction existed between the body cavity and digestive 

 tract. In the animals higher than the ccelenterates the digestive 

 tract and body cavity are distinct. The digested food material 

 passes by osmosis into the body cavity. Some food reaches the 

 blood vessels and is pumped over the body, most of it is used by 

 the organs which it bathes. Nitrogenous waste is excreted from 

 each segment through a pair of coiled tubules called nephridia. 



Course of Blood. — In a large earthworm the course of the 

 blood is easy to follow. Notice the position of the dorsal blood 

 vessel. Watch it at one point. The blood vessel expands as the 



