86 THE BIOLOGY OF MARINE ANIMALS 



body fluids), the total extracellular space in mammals is found to lie around 

 20-30 % of body weight. About one-third of this volume is occupied by 

 blood; the remainder is lymph and interstitial fluids. In the mussel Mytilus 

 edulis, intercellular fluid or haemolymph is estimated to be about 12%; 

 the extracellular space in crabs Carcinus and Eriocheir forms 33-37 % of 

 body weight. Haemolymph volume in these animals is equivalent to total 

 extracellular space in vertebrates. In cephalopods, which also possess a 

 closed vascular system, total extracellular fluid as measured by sucrose 

 injection amounts to about 33% (78, 114). 



In the open circulatory system of the mussel, the haemolymph follows 

 alterations of the external medium fairly closely. Mytilus califomianus 

 maintains a constant ratio of about 1-6-1-0 between Cl~ concentrations 

 of external medium and intercellular fluid over a restricted environmental 

 range (CI - concentrations of 0-9-2-8%). Within these limits, Cl~ exchange 

 occurs only between haemolymph and external medium. Beyond this 

 range, however, Cl~ exchange takes place between the cellular contents 

 and the haemolymph, and the cells are unable to maintain a steady internal 

 concentration in the face of marked changes in external chlorinity. Where 

 a well-developed closed circulatory system is present, as in vertebrates, the 

 tissue fluids form a reservoir of water and salts which can be called upon 

 to buffer alterations in the composition of circulating fluids (40, 78). 



Fluids in Primary Body Cavities. Many invertebrates retain the primary 

 body cavity or blastocoele in the adult as spaces of various extent filled 

 with intercellular fluid. In the preceding section reference has been made 

 to the relationship between interstitial and circulatory fluids, and we have 

 noted that in various primitive groups all the body fluids of the organism 

 can be regarded as a continuous medium occupying primary body spaces, 

 sinuses and intercellular meshes. In platyhelminths the parenchyma con- 

 tains large intercellular spaces filled with tissue fluid. In nemertines the 

 diffuse body cavity is reduced to a system of one or more definite vascular 

 channels. Longitudinal vessels are connected together by contractile trans- 

 verse vessels and these, aided by body movements, circulate the contained 

 fluids. This is the beginning of a true circulatory system concerned, 

 among other things, with transport of oxygen to more deeply lying 

 tissues (27). 



In those animals with open circulatory systems (Mollusca, Crustacea, 

 Xiphosura) the primitive body cavity is retained in large haemocoelic 

 spaces through which the blood or haemolymph slowly passes on its way 

 back from the tissues to the heart. 



Coelomic Fluids. Extensive coelomic spaces filled with fluid are found in 

 polychaetes, sipunculoids, echiuroids, ectoproct polyzoans, phoronids, 

 chaetognaths and echinoderms. In leeches the coelomic spaces are exten- 

 sively invaded by mesenchymatous tissue and are reduced to a system of 

 longitudinal channels. In molluscs the coelomic spaces are restricted to 

 small cavities in the kidneys, gonads and pericardium. A similar condition 

 obtains in crustaceans. Coelomic cavities are, of course, well developed in 



