90 THE BIOLOGY OF MARINE ANIMALS 



consist of two epithelial layers; capillaries and lacunae contain only a 

 single layer of endothelium. Nemertines, the most primitive animals to 

 possess a closed vascular system, have two kinds of vessels. The larger 

 contractile vessels are provided with a wall of four layers, namely lining 

 endothelium (flat or bulging cells), gelatinous connective tissue, circular 

 and longitudinal muscles, and an outer non-nucleated covering. Smaller 

 non-contractile vessels lack the muscle layer and consist of inner and outer 

 epithelia, with intervening membrane (27, 64, 109). 



Among other invertebrates with closed circulatory systems the cephalo- 

 pods are worthy of special attention. There is a rich peripheral capillary 

 network; arteries and veins are provided with striped muscle, which is 

 hypertrophied in arterial and branchial hearts (121). 



The vascular systems of lower chordates and invertebrates invite further 

 investigations from many aspects. For each of the major groups in which 

 the systems are closed it is desirable to obtain a sound histological picture 

 of the structure of the vascular walls, and to relate this to permeability, 

 inherent contractility, maintenance of tonus and nervous regulation. 

 Partition of fluids and alterations of volume with changing functional 

 states are obvious physiological variables. In the case of open systems 

 there is much to be learnt about volume changes in different sinuses, 

 participation in hydraulic mechanisms and pressure changes under differ- 

 ent functional conditions. In all cases these factors have to be related to 

 the output of the heart and the contribution of contractile vessels as main 

 or subsidiary pumping agencies. 



Pressure in Circulatory Systems 



Blood is propelled through vascular channels by the pumping action of 

 special hearts and contractile vessels, aided in many animals by somatic 

 movements. The pressure head which is built up by the pumping system is 

 gradually dissipated in the vascular channels through frictional losses. 

 The level of blood pressure is determined by the volume of circulating 

 fluid, the force exerted by the heart or other contractile structure and by 

 the peripheral resistance. 



Closed Circulatory Systems 



Fishes. Haemodynamics and regulation of blood pressure have received 

 relatively little attention in lower animals, and data are insufficient for a 

 satisfactory comparative treatment. Pressure regulation in fishes provides 

 an interesting contrast with regulation in mammals. Some selected values 

 for blood pressures in arterial vessels of fishes are given in Table 3.2 

 (26, 34, 58, 62, 81, 82, 108). 



In fishes the blood which leaves the heart passes through the branchial 

 vessels before reaching the dorsal aorta. Interposition of the branchial 

 capillaries produces a pressure drop before the blood actually reaches the 

 systemic circulation, and pressure continues to drop in peripheral vessels. 

 Pressures are lower and circulation more sluggish in fishes than in homoeo- 



