CLAMS AND OTHER MOLLUSKS 343 



food-distributing system. Such an apparatus is a true 

 adaptation for respiratory purposes. 



Worms. — The worms obtain their oxygen supply by 

 absorbing it from the air or water through the skin, which 

 is thin and moist, and in direct contact with 

 the air or water in which they hve. In some f'\ 

 cases this skin is produced into pouches or tufts 

 in order to increase the respiratory surface, and 

 such tufts are called gills. After the oxygen 

 has passed into the body cavity through the 

 gills it must be distributed to the tissue cells. 

 In some worms this distributing is performed 

 by the digestive fluids. In others, such as the 

 earthworm, this oxygen passes through the thin 

 walls of a blood tube where it unites with 

 haemoglobin dissolved in the blood fluid, and, 

 in this case, the blood acts as a true distribu- 

 tor of the gas to the cells. This blood also 

 collects the carbon dioxide, which is passed 

 out through the skin in the same manner that 

 the oxygen enters it. 



In the earthworms we find the first ex- 

 ample of the blood acting in connection with 

 the skin as a medium for the distribution of "TheTob- 

 oxygen and carbon dioxide. It is to be noted ZVJ^- 

 that in this case the skin is the respiratory organ ingtutt- 

 and the blood plasma the distributor, since the 

 blood contains no corpuscles. In order to absorb oxygen 

 the skin must be very thin and moist. 



Clams and other moUusks. — In the clam and the oyster 

 there is a heart and a definite system of branching blood 

 vessels. Some of the smaller branches pass into four flaps 



