DEVELOPMENT OF BLOOD 299 



amount of traffic on this system to some extent controls the rate 

 of importation, it will be convenient to direct attention to it in the 

 first place. 



Inland Transport — The Blood 



The blood has been called the liquid tissue of the body. On 

 two counts this is a misnomer. Firstly, it tends to detract from 

 the liquidity of the tissues in general. Further, blood cannot 

 rightly be considered a tissue at all. No doubt a very pretty 

 picture could be drawn of blood and its containing membranes 

 as a tissue, clotting, as other tissues clot, on death, but when the 

 facts are examined they do not bear out such an idea. The 

 evidence too, culled from comparative studies of the development 

 of a circulatory system, is all at variance with the liquid tissue 

 theory. 



1. Development. 



Much may be learned from a study of the evolution of 

 any system. Material exists for such a study in Comparative 

 Physiology. 



{a) Unicellular organisms require no circulatory system. Their 

 imports go direct to the sole factory of the place. They may be 

 landed at any part of the coast and are at once acted on. What 

 is suitable is accepted, the residue is rejected or left untouched. 

 Examination of a unicellular animal leads to the conclusion that 

 the cell contents are in a state of constant motion. Water, every 

 now and then, is engulfed, passes more or less directly through 

 the organism, and is excreted, carrying with it the by-products 

 of cell activity. 



{b) Some invertebrates have an open coelomic system. Their 

 more complex structure necessitates the production of a current 

 of fluid so that material may reach the inner cells. That is, some 

 of the water in which the animal lives is passed by means of canals 

 to the different parts of the body. The fluid is kept in circulation 

 by the rhythmic contractions of whip-like processes called cilia {q.v.) 

 The ciliary waves force the water through the tubes into the 

 lacunae of the tissues. Such a system is difflcult to control. It 

 is dependent on the nature of the bathing medium. It carries the 

 possibility of constant changes in the salt content of the cells of 

 the animal. Any change in the environment will be passed on 

 to the coelomic fluid and at once reflected in the cell. It demands 

 constant adaptation on the part of the organism, and thus it is not 

 economical (cf. our system of canals). 



(c) Higher invertebrates and the vertebrate amphioxus have 



