102 THE BIOLOGY OF MARINE ANIMALS 



consequence of these various factors the velocity of circulation is rapid, 

 but the actual circuit-time has never been measured. 



Turning now to the properties of molluscan heart muscle, we find cer- 

 tain differences from vertebrate cardiac muscle. The molluscan heart is 

 excitable at all stages of the cardiac cycle, but threshold is high during 

 systole, and a strong stimulus is required to elicit a contraction. A condi- 

 tion approaching absolute refractoriness exists during early systole, after 

 which excitability gradually returns (Fig. 3.6). It follows from these charac- 

 teristics that the molluscan heart can be tetanized by repetitive stimulation. 



Fig. 3.6. Effect of Electrical Stimuli on 

 Ventricular Contraction of Aplysia 



Stimulus ineffective during systole (a), but effective during diastole (b), and diastolic 

 pause (d). (From Carlson, 1906.) 



The frequency and force of contractions are dependent on the internal 

 pressure (cf. Starling's law of the vertebrate heart), and some molluscan 

 hearts fail to beat unless sufficiently distended. The heart of Octopus 

 ceases to beat when the internal pressure falls below 2 cm H 2 0, and the 

 frequency increases with rise in pressure (26, 77, 121). 



Fishes. The fish heart is derived during ontogeny from a tubular struc- 

 ture and consists of three successive chambers, namely sinus venosus, 

 auricle and ventricle. These are guarded by valves: S.V. (sinus venosus) 

 valves where the ducts of Cuvier enter the sinus, S.A. (sino-auricular) 

 valves between sinus and auricle, and A.V. (auriculo-ventricular) valves 

 guarding the auriculo-ventricular junction. The ventricle leads into a 

 muscular and contractile truncus arteriosus in elasmobranchs, and a 

 fibrous bulbus arteriosus in teleosts. These latter structures are provided 

 with semilunar valves. The heart discharges into a ventral aorta from which 

 afferent branchial arteries carry the blood to the gills before it enters the 

 systemic circulation. 



The sequence of cardiac contraction is from sinus to ventricle. The sinus 

 and auricle have thin muscular walls, especially the former, and are essen- 

 tially receiving chambers; the ventricle has strong muscular walls and is 

 the effective pump. The valves are arranged so as to prevent reflux of 

 blood, and close when the pressure on the outgoing side exceeds that on 

 the incoming side. Conditions affecting venous return in fishes are poorly 

 understood; venous pressures measured near the heart lie around zero, 

 and it is likely that swimming movements and contractions of the body 

 wall are important in moving blood back to the heart. The sinus fills 

 during diastole, and when contraction ensues the S.A. valves open and the 

 auricle fills. As the auricle fills its pressure rises and during systole the S.A. 

 valves close, the A.V. valves are forced open and blood fills the ventricle. 



