104 THE BIOLOGY OF MARINE ANIMALS 



heart beat up to frequencies of 45-50 beats per min, beyond which there 

 would appear to be little opportunity for further increase of pressure. As 

 the heart rate increases, pulse pressure declines, while systolic pressure 

 rises. 



The output per heart beat is known as the stroke volume. The stroke 

 output of teleosts varies with the weight of the fish. For the eel Anguilla 

 bostcniensis it ranges from 0T2 g for a 200-g fish, to 0-30 g for a 600-g fish. 

 The cardiac output (stroke volume) of the dogfish Squalus acanthias is 

 0-4-1-5 c.c. for a normal animal weighing 1,600 g, with a maximal value 

 of 3 c.c. At a cardiac frequency of 36 per min this gives an average minute 

 volume of about 20 c.c per kg fish. Heart frequencies of some representa- 

 tive species of selachians and teleosts are given in Table 3.3. These show 

 about the same range as in cephalopods, invertebrates of comparable size 

 and blood pressures (19, 119). 



Tubular Hearts 



Hearts of arthropods, when present, usually take the form of contractile 

 tubes or are derived therefrom. In Crustacea the heart lies in a large peri- 

 cardial sinus, with which it communicates by several ostia guarded by 

 valves. Isopods and amphipods have long tubular hearts (Fig. 3.8). In 

 decapods they are polygonal-shaped chambers, lying freely in the peri- 

 cardium and suspended at several corners by strands (Fig. 3.17). Blood is 

 discharged from the heart through several arteries (five in front and two 

 behind in Homarus), which are also supplied with valves. The arteries carry 

 blood to all parts of the body; after bathing the tissues the venous blood 

 passes by a lacunar system into a large ventral sinus, whence it is carried 

 by afferent branchial vessels to the gills. In the latter there is a complicated 

 through circulation by which venous blood is brought close to the surface 

 of the gill filaments in distinct afferent and efferent capillaries. From the 

 gills the blood passes by efferent branchio-cardiac veins to the pericardium, 

 and thence to the heart. The heart of Limulus is a long segmental tube 

 perforated by eight pairs of ostia, which mark it off into eight segments 

 (Fig. 3.9). In the anterior half are five pairs of arteries plus one antero- 

 median artery. A receiving chamber ("auricle") covers the posterior region 

 of the heart. Arthropod heart muscle is cross striated, and consists of 

 circularly or spirally arranged fibres. There is some doubt whether the 

 heart muscle is truly syncytial. 



Arthropod heart muscle resembles the molluscan heart in its contrac- 

 tile properties. There is a condition of reduced excitability early in systole, 

 after which excitability gradually returns (Fig. 3.10). This condition is 

 relative, however, and by increasing the strength of stimulus a contraction 

 can be produced at any stage of the cardiac cycle. At slow frequencies of 

 stimulation (1 per sec), staircase, or progressive increment of consecutive 

 responses, can be demonstrated in crab heart (Cancer). 



An investigation of haemodynamics in the lobster (Homarus americanus) 

 shows that the cardiac stroke volume is about 0-1-0-3 c.c. An animal hav- 



