BODY FLUIDS AND CIRCULATION 87 



the lower chordates, and coelomic fluid occurs in the spaces between the 

 body wall and the internal organs. 



Blood in Closed Vascular Systems. Closed vascular systems are found in 

 the following marine groups: nemertines, polychaetes, leeches, sipuncu- 

 loids, phoronids, cephalopods, holothurians and chordates. Usually such 

 systems contain a relatively small volume of blood which is pumped con- 

 tinuously around the organism. The hearts or other devices which provide 

 the motive force for propelling the body fluids display great variation in 

 structure and efficiency, and some are subject to various degrees of nervous 

 control. 



Lymphatic systems resembling those occurring in vertebrates are not 

 encountered among invertebrates. Gnathobdellid leeches, however, are 

 peculiar in having blood vessels communicating with coelomic sinuses and 

 with intracellular capillaries of botryoidal tissue in a manner analogous 

 with the lymphatic vessels of vertebrates. 



In the following pages attention will be restricted to fluid systems of 

 marine invertebrates and the lower chordates. For a comparative account 

 of the physiology of circulatory systems see Scheer (116), Prosser (108) and 

 Carter (21). 



VOLUME OF CIRCULATING FLUIDS 



In higher animals with closed circulatory systems the blood forms only a 

 small and relatively fixed fraction of the body weight. Although much 

 attention has been devoted to blood volumes of mammals, especially man, 

 very little information is available for invertebrates, and it would be valu- 

 able to have more comparative data and information dealing with body 

 fluids in lower forms. 



Observations dealing with blood volumes of various animals have been 

 summarized by Reichert and Brown (113) and Prosser (108), and some 

 selected values are given in Table 3.1. These have been obtained by several 

 methods. One consists in determining the haemoglobin content of a sample 

 of blood, and then bleeding the animal and washing out the vessels to 

 obtain the total haemoglobin in the body ; with these data the blood volume 

 is computed. Others involve addition of a known quantity of some sub- 

 stance which remains confined to the blood stream, and determining the 

 degree of dilution which takes place after it becomes uniformly distributed 

 in the circulation. For this purpose dyes are usually employed (Evans' blue, 

 vital red). Total extracellular fluid has been estimated by using substances 

 such as thiocyanate and sucrose, which are not taken up by tissue cells. 



In fishes, with closed circulatory systems, blood volume is small (1-5-4% 

 in teleosts; 4-18% in selachians, with majority of values below 7%). 

 These values contrast with the large blood volumes found in invertebrates 

 with open circulatory systems (17-37% in crustaceans; 8-13% in lamelli- 

 branchs). Utilization of a small volume of circulatory fluid is more efficient 

 than a large one, when it is pumped around the body faster and made use 

 of more often. In these terms the blood circulation of a fish is more 



