132 THE INVERTEBRATA 



it has usually the form of a branching system of vessels (''vascular 

 system") through which the fluid is caused to circulate by the con- 

 traction of muscular fibres in the wall of some portion of it which is 

 known as a heart. In some cases, however, the haemocoele forms 

 large "perivisceral" sinuses around the internal organs. It never 

 contains germ cells or communicates with the exterior. 



Since the haemocoele fluid is in intimate relation with the tissue, 

 its composition is a matter of very great importance to the animal. 

 It bears to the tissues much the same relation that the external 

 medium bears to the body as a whole and is on that account often 

 spoken of as an internal medium} If it be changed the working of the 

 organism is influenced. It is liable to be fouled by poisonous waste 

 products of metabolism and these must be removed from it and ex- 

 creted or so changed as to be harmless. It is liable to alteration by 

 diffusion between it and the external medium, and in proportion as 

 this can take place the animal will be at the mercy of its surroundings. 

 To maintain it in a constant condition in respect of the substances 

 which it might exchange with a particular external medium two 

 agencies are at work — the guardianship, active or passive, of the pro- 

 tective sheet of ectoderm and of any cuticle or other covering which the 

 latter may secrete, and the activity of the excretory organs, especially 

 in the excretion of water. The effectiveness of these agencies varies. 

 The independence of the body fluids from the external medium is 

 least in some marine animals, such as echinoderms and certain 

 molluscs: in these the fluids closely resemble sea water both in the 

 ions present and in the total osmotic pressure. In a series of others, 

 independence grows, and it is highest, in the sea, in teleostean fishes. 

 In fresh water animals the composition of the blood is kept entirely 

 different from that of the external medium. In land animals there is 

 of course no question of the exchange of solutes, and unless the loss 

 of water were reduced to a minimum life would be impossible. It is 

 an interesting fact that, though the resemblance of the body fluids 

 in fresh water and land animals to sea water is much less than that 

 of marine animals, something of it still remains, no doubt because 

 protoplasm came into being in sea water and still requires to be 

 bathed by a fluid which somewhat resembles the latter. The principal 

 differences are an increase in potassium and a decrease in magnesium 

 and SO4 ions and a lower total osmotic pressure. 



The blood is the principal means of transport within the body. 

 A very important part of its freight is oxygen. Its capacity for 



^ The fluids of the secondary body cavity (coelomic fluids) are also internal 

 media, but less intimate and therefore chemically less important than the 

 blood. In echinoderms, tiowever, they are probably more important than the 

 fluid of the vestigial haemocoele (lacunar system, p. 629). 



