jgQ THE CCELOMATE BODY 



chinks between the cells of the embryo, and the coelom the secon- 

 dary body cavity. Since in many embryos this temporal dis- 

 tinction cannot be made, and since neither is ever present in 

 an adult animal without the other and they add nothing in 

 meaning, these terms are best abandoned. 



FUNCTIONS OF BLOOD 



The properties of the blood, the fluid and cells present in the 

 blood-vascular system, are usually listed as many, but all the 

 important ones can be summed up in one word : transport. The 

 mesoderm enables an animal to become large ; bulk prevents 

 easy access to the cells by simple diffusion across adjacent cells, 

 and so a transport system becomes necessary. The things which 

 are carried to the cells are oxygen, either in solution or in chemical 

 combination with a substance such as haemoglobin ; food, chiefly 

 as amino-acids and hexoses, and hormones (p. i8). These last are 

 known in vertebrates, cephalopods, insects, crustaceans and 

 annelids, that is, in all the chief phyla with blood. From the cells 

 the blood takes excretory products, including carbon dioxide, and 

 heat. In large animals the latter is very important, for a cell in 

 the middle of a big mass of muscle is well insulated, and if there 

 were no cooling system it would overheat. The fact that in insects, 

 where oxygen is carried by different means, the blood system is 

 far less well developed than it is in other arthropods, suggests that 

 oxygen-transport is the most important function of the blood. 

 Besides carrying substances from one place to another, the blood 

 is also important in forming a relatively stable internal environ- 

 ment for the active body cells. This environment is an aqueous 

 solution in which the main constituents are kept at an approxi- 

 mately constant composition ; the hydrogen ion concentration of 

 vertebrate blood does not greatly fluctuate, even though large 

 quantities of lactic acid and carbon dioxide are put into it, because 

 it is well buffered with phosphate. Other ions, and substances 

 such as glucose, are kept constant by the co-operation of the 

 kidney, which eliminates them if they are present in excess and 

 holds them back if there is not enough of them. Other excretory 

 organs, such as the nephridia of the earthworm and the green 

 glands of the crayfish, act in a similar, but probably less elaborate, 

 way. 



These are the fundamental functions of blood, which would 



