94 THE BIOLOGY OF MARINE ANIMALS 



Open Circulatory Systems 



Animals with open circulations have low and highly variable blood 

 pressures. In small crustaceans — e.g. cirripedes, many copepods and 

 ostracods — a heart is often wanting and blood is circulated solely by 

 movements of the body wall and alimentary canal. Increase in size and 

 activity is attended by development of a heart pump, but even in large 

 crustaceans the pressure head developed by the heart is low, and circula- 

 tion of the blood owes much to movements of the appendages and the 

 body wall. Recorded blood pressure in the sternal sinus of the shore crab 

 Carcinus maenas, a relatively small but active animal, is 13 cm H 2 0. In 

 large specimens of Maia, a relatively sluggish crab, systolic pressure in the 

 heart is 55 mm H 2 0, and the difference between systolic and diastolic 

 pressures is 8 mm H 2 0. Pressure drop from the arteries to the thoracic 

 sinus is about 25 mm H 2 (absolute pressures in the latter 24-34 mm H 2 0). 

 In the lobster (Homarus), a large active animal, the intraventricular blood 

 pressure is 17-7-1-36 cm H 2 0, and the aortic pressure immediately 

 posterior to the ventricle is 17-7 cm during systole and 12 cm at diastole. 

 Pressures in the haemocoele range from 27-82 mm H 2 0; pericardial pres- 

 sures are the lowest of the circulatory system, 0-16 mm H 2 0. Vascular 

 pressures vary with activity of the heart, with body tonus and body move- 

 ments. In decapod crustaceans blood pressures in arteries and the haemo- 

 coele rise greatly during activity (20). 



An open circulatory system is also characteristic of gastropods and 

 lamellibranchs. Arterial pressures in freshwater mussels (Anodonta) are 

 lower than in decapod crustaceans, about 35 mm H 2 at systole, falling to 

 10 mm H 2 during diastole. Muscular contraction, causing ejection of 

 water from the exhalant siphon, doubles arterial pressure. Pressures in the 

 haemocoele of the sea hare Aplysia, a gastropod, are much above those 

 of Anodonta, and approximate those of decapod crustaceans. Pressures in 

 the body cavity of a resting animal were 2-5-4 cm H 2 0, and rose to 6 cm 

 during activity. 



The heart is a weak and rather inefficient pump in crustaceans and 

 molluscs with open circulatory systems. Some of the work involved in 

 moving the blood about the body is performed by the somatic musculature 

 through changes in body tonus and activity; during periods of higher 

 activity, when oxygen requirements are greater, blood pressure and circu- 

 latory movements are automatically increased (26, 102, 108, 121). 



HYDROSTATIC PRESSURES IN BODY CAVITIES 



Fluids in the body cavities of many soft-bodied animals, as we have 

 already noted, have several functional roles including circulation of 

 essential substances, removal of wastes, provision of a constant internal 

 milieu and participation in hydraulic mechanisms essential for movement 

 and locomotion. 



In lamellibranchs in which the foot is employed for locomotion and 



