Chap, v.] THE BLOOD. 181 



No Vertebrate is truly parasitic, for Myxine 

 (the " Borer ") penetrates the body of other fish for 

 the direct purpose of feeding on their flesh ; and the 

 relation of Fierasfers to the Medusae, Echinoderms, and 

 Molluscs, with which they have been found, is only 

 that of a guest which makes use of the currents of 

 water which the host intended for its own purposes 

 (SYMBIOSIS). 



CHAPTER V. 



THE BLOOD AND THE BLOOD-VASCULAR SYSTEM. 



THE result of the process of digestion is the formation 

 of a quantity of material which can be usefully taken 

 up by the different cells of which the body is com- 

 posed, and used by them for the purposes of repair, 

 growth, and reproduction. In a majority of cases the 

 material has yet another function, inasmuch as it 

 becomes the vehicle for the oxygen which is constantly 

 necessary to cell-activity ; it is respiratory as well 

 as nutrient. 



This material takes the form of a liquid in which 

 cells or corpuscles float ; and these cells are either 

 colourless, or tinged red by haemoglobin, and 

 are either amoeboid or constant in form. The 

 liquid is known as the plasma, and it is either 

 colourless, or tinged red, green, or blue. 



The different characteristics of the various parts of 

 this nutrient and respiratory medium are most clearly 

 seen in the Yertebrata, with which, therefore, we will 

 commence. 



The blood is a fluid containing white and red cor- 

 puscles, a certain amount of dissolved albuminous 

 and mineral bodies, and about half its own volume of 



