FUNCTION OF BLOOD 231 



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 

 a closed system in which the fluid passes through tubes capable 

 of rhythmic contractions. 



(d) The vertebrates have a closed vascular system with the 

 advantages of ease of control and freedom from constant adapta- 

 tion. It is the most economic system of transport known. 



When the sea animals crawled on to land and became breathers 

 of air, they included a certain proportion of the sea- water in their 

 vessels. By the alteration in surface tension caused by the 

 exchange of a protoplasm-water interface for a protoplasm-air 

 interface their open coelomic system automatically closed (cf. 

 camels' hair brush experiment, p. 403). 



The vertebrate has, therefore, a fluid in its vessels having a 

 composition similar to that of the sea from which originally it 

 came (see salts of plasma, p. 239). Some arose from a tropical 

 sea and therefore this included fluid was warm. Others again 

 did not attain to this. We have, therefore, warm and cold 

 blooded animals (Regulation of temperature, Chap. XXXI.). 

 This is a very pretty theory. It cannot be considered as proved 

 any more than the hypothesis of evolution, but in the same sense 

 both fit in with certain facts. 



2. Function. 



(a) The blood-stream conveys food including oxygen to all 

 parts of the organism. 



(b) It removes the waste products of activity including carbon- 

 dioxide, which would paralyse function if allowed to accumulate. 



(c) The carriage of chemical substances (hormones) from the 

 organs in which they are produced in order to influence the 

 activity of other organs may be considered as the co-ordinative 

 action of the circulation. 



(d) The movement of blood aids in the regulation of the tem- 

 perature of the body (Chap. XXXI.). 



(e) It plays a very important part in the defence of the 

 organism against parasites, etc. 



(/) The preservation of the H-ion concentration of the body 

 is principally a function of the circulating fluid (Chap. XXX.). 



(g) It maintains the water and salt content of the body at a 

 certain level. 



