Circulation of Body Fluids 541 



between animals with open and animals with closed circulations is the low 

 and variable blood pressure in the animals with open systems. One oF the 

 most active of the large animals with an open circulatory system is the 

 lobster; pressure in its dorsal artery is 5-17 (average 11.5) cm. H2O, with 

 a pulse pressure of 10 mm. H^O.-'' Pressure in the sternal sinus of 

 Carcinus is 13 cm. HoO.^'^- In large specimens of Maja the arterial pressure 

 is 45-52 mm. HjO, whereas pressure in thoracic sinuses is 25-34 mm. H^O.-'*** 



Pressure regulation and directions of blood flow in open vascular systems 

 are poorly understood. In all animals in which the blood circulates in 

 sinuses the flow is controlled much more by the activity of somatic muscul- 

 ature than by heart activity and vessel size. Some small crustaceans and 

 insects lack a pulsating heart. Septa separate afferent and efferent channels 

 in gills (crayfish and lobster"^-*) and in legs (dragonfly nymphs^"*'^). 



In some insects the heart is connected to the dorsal wall by alary muscles, 

 which stretch the heart muscle. It has been suggested that the heart has an 

 aspirator action, and that the pressure in the hemocoel is negative. ^^' ^^^ A 

 drop of fluid on a cut appendage is sucked inward in many insects. ^^' ^* 

 However, in dragonfly nymphs the pressure in the body cavity is negative 

 when the animals are anesthetized but not when they are active.^^' In a 

 series of measurements on crayfish about 10 per cent of the animals were 

 very lethargic, and, in these, negative pressures were recorded at the heart 

 as well as in leg sinuses.^^^ Pressure may be negative in extreme relaxation 

 but mav increase by many times on motor activity. In dragonfly nymphs 

 the pressure in the abdomen rises and falls with movements as in breath- 

 jj^g 17:^, i:s7 jj^ fj-^g crayfish the pressure in the hemocoel at the distal end of 

 a leg is often higher than at the proximal end, and the pressure in the 

 cheliped (average proximal, 8-6 cm. H2O; distal, 10-8 cm. H2O) may be 

 higher than in walking legs.^*^^ Simultaneous measurements in several 

 regions showed that the pressure increases with activity are not the same 

 throughout the hemocoel. Pressure in the heart exceeds pressure in the 

 legs except in activity. When pressure in an appendage rises because of 

 muscular activity there may be backflow of blood or blood may be confined 

 by valves at the joints of appendages. In Maja thoracic sinus pressure 

 increases with leg activity. ^^ In fresh-water mussels the pressure in the 

 heart and pericardial sinus is low (Table 67), but when the body contracts 

 during ejection of water through the excurrent siphon, the pressure in the 

 sinus and heart doubles. ^•'^ 



In animals with an open circulatory system the blood flow in an organ is 

 more directly a part of "voluntary" activity and pressure changes are many 

 times greater than in animals with closed vessels. In molluscs and crusta- 

 ceans the blood volume is probably comparable to the volume of total extra- 

 cellular fluid of a vertebrate, yet the blood pressure of a 20 gram crayfish 

 is only about 30 per cent of that of a 20 gram mouse. The velocity of circu- 

 latory flow in the crayfish is, therefore, very low; the sluggishness of blood 

 flow may be a limiting factor in the size of animals with an open circulation. 



In many tubular animals the pressure in fluids of the body cavities is 

 dependent on tone of the body wall (holothurians, annelids— Table 67). In 

 such animals hydraulic mechanisms are important in other respects than the 



