110 



THE BIOLOGY OF MARINE ANIMALS 



Raja) (Fig. 3.13). There is occasionally an initial inhibitory effect, which is 

 blocked by atropine. The effect of these amines on the teleost heart is 

 slight (6, 19, 60, 97). 



Molluscan hearts are myogenic, and all regions can show autonomous 

 contractions. During contraction there is a well-marked A-V interval, 

 amounting to about 0-5 sec. Electrocardiograms have been recorded from 

 several mulluscan hearts, and show the slow waves characteristic of myo- 

 genic types. In Octopus there is an initial fast deflexion succeeded by a 

 prolonged wave of negativity, and the same sort of pattern is shown by the 

 gill heart of Lo/igo. The electrocardiogram of Aplysia consists of slow 

 waves with superimposed irregular deflexions; those of bivalves (Ostrea, 

 Anodontd) normally display a diphasic component near the beginning of 



-*&-*&- 



(a) (b) 



Fig. 3.14. Electrocardiograms of Molluscan Hearts 



(a). Aplysia. From above downwards, myogram, electrocardiogram, time in seconds 

 (from Hoffmann, 1911.) 



(b). Crassostrea virginica. Electrical record, small waves; mechanical tracing, large 

 waves. Upward deflexion of myogram indicates contraction. Time scale below, 1 sec. 

 Temp. 22°C (from Taylor and Walzl (126).) 



contraction and one or several slow waves associated with contraction 

 (Fig. 3.14). Apparently the fast component in the ECG represents the 

 spread of excitation; and the slow waves, potential changes taking place 

 during contraction. 



Although showing automaticity, the hearts of molluscs are subject to 

 nervous regulation. Cardiac nerves arise from the visceral ganglion in 

 lamellibranchs ; the presence of nerve cells in the heart is disputed. In 

 gastropods cardiac nerves arise from the visceral or accessory visceral 

 ganglion, and terminate in auricle and ventricle. The existence of nerve 

 cells is in doubt; if present they probably represent secondary regulatory 

 neurones. The nervous supply of the cephalopod heart has been traced in 

 detail. In these animals the cardiac branches arise from the visceral nerves 

 and proceed to the heart via ganglia. From the latter, branches pass to 

 auricles, ventricle and branchial hearts. 



Inhibitory and acceleratory cardiac fibres have been identified in certain 

 molluscs. Inhibitory nerves are particularly in evidence among lamelli- 

 branchs but are also recorded for certain gastropods (nudibranchs and 

 pulmonates). Stimulation of the visceral nerves and ganglia causes well- 

 marked inhibition of the lamellibranch heart (Fig. 3.15). Acceleratory 



