ORGANIC EVOLUTION 177 



metabolic processes oxygen is also necessary, and this the 

 worm gets after a very primitive fashion by direct absorp- 

 tion through the skin. To facilitate this the skin is kept 

 moistened with the watery mucous poured out upon it by 

 numerous secreting cells in the epidermis (fig. 104 b, h). 



The red color of the blood is due to the presence in it of 

 haemoglobin, a substance that is an excellent agent for the 

 transport of oxygen. It combines loosely with free oxygen, 

 taking it up readily where there is a copious supply, as at 

 the skin, and giving it up easily, where affinities for it are 

 stronger, as in the active and deoxydized tissues. The blood 

 is, therefore, the carrier of both food and oxygen to every cell. 

 It carries the former outward from the stomach wall, the 

 latter inward from the skin. 



Income and outgo are not essentially different for any cell 

 in the worm's body from the same process in the protozoan 

 cell as outlined on p. 91. More food passes through the 

 epithelial cell and more oxygen through the epidermal cell 

 than through the others, as more stores pass through the 

 seaport towns of an importing country than through the 

 interior ones ; but the part reserved by each for its own use 

 is used by all in much the same way. The output of matter 

 is therefore, as in the protozoan, mainly water, carbonic acid 

 gas and simple nitrogen compounds. 



These waste products must be gotten rid of, and while 

 the cells of the surfaces of the body may excrete them directly 

 to the outside, those of the interior need the circulatory 

 system to carry off their waste. The dispersal of the carbon 

 dioxide and water is as general over the surface of the body 

 as the intake of oxygen. With these doubtless goes a part 

 of the nitrogen waste also. But the nephridia are special 

 agents for disposal of the nitrogen waste. To these the 

 blood goes, laden with the products of proteid dissimilation, 

 and in them these substance are removed and passed to the 

 exterior. 



