BODY FLUIDS AND CIRCULATION 89 



efficient than that of the crab (open system). But the haemolymph of the 

 latter is really equivalent to blood plus interstitial fluids of animals with 

 closed circulatory systems, and a comparison on a basis of this kind is 

 difficult. The values for Arenicola (minimal estimates) show that large 

 blood volumes are not confined to animals with open circulatory systems. 

 In addition to volumetric considerations, overall efficiency will also depend 

 on hydrodynamic factors, especially the configuration of the peripheral 

 bed and the mechanics of the pumping system. The amount of coelomic 

 fluid in some of the soft-bodied lower invertebrates appears to be very 

 high. Thus, in echiuroids and priapuloids it forms 30-50% of body weight 

 or volume. 



BLOOD VESSELS 



In those animals with open circulatory systems the blood or haemolymph 

 comes into direct contact with the tissue cells. When a closed vascular 

 system is present the blood constituents must diffuse across the walls of 

 sinuses, small capillaries, etc., to reach the tissue fluids and cells. In the 

 tissues the blood unloads gases, nutrients, etc., and picks up metabolites 

 originating in the tissue cells. The composition of the blood is restored by 

 exchanges which take place in the circulatory bed of skin, gills, excretory 

 organs, etc. 



In vertebrates the walls of the larger vessels are composed of layers of 

 muscular and connective tissue (elastic and collagen fibres), with an in- 

 ternal lining of endothelium. Changes in diameter of these vessels serve 

 to accommodate alterations in blood volume, either locally or widespread, 

 and changes in vascular tonus participate in regulation of blood pressure. 

 The capillary wall is a semipermeable membrane, freely permeable to 

 water, salts and small organic molecules. Temporary gaps between the 

 endothelial cells allow the escape of colloidal particles (protein, particulate 

 matter) and occasional blood cells. The residual osmotic pressure, exerted 

 by plasma protein, counterbalances in large part capillary hydrostatic 

 pressure. Colloidal osmotic pressures in the plasma of marine teleosts lie 

 between 4-28 cm H 2 0. Values are much lower in elasmobranchs, ranging 

 from 17-46 mm H 2 (70, 128). Pressure-drop along the capillary results 

 in some degree of circulation in the tissue spaces, fluid escaping from the 

 arterioles and capillaries proximally returning again at the venous end. 



Among invertebrates muscular blood vessels occur in many groups, but 

 little is known of the role they play in regulation of circulatory conditions. 

 The histology of vessels has been most intensively investigated in the 

 Annelida. Generally these consist of endothelial, skeletal and peritoneal 

 layers. The endothelium is discontinuous and is made up of flat branched 

 cells. The skeletal coat consists of collagenous material. Muscle fibres are 

 frequently present in the external peritoneal layer (55, 56, 96). 



In holothurians the larger haemal vessels (intestinal vessels) have rela- 

 tively thick walls composed of internal endothelium, loose connective 

 tissue and muscle, and an external peritoneum. Vessels of the rete mirabile 



