116 THE BIOLOGY OF MARINE ANIMALS 



each burst determines the amplitude of contraction. Of the various kinds 

 of neurones in the cardiac ganglion, the small cells behave as pacemakers 

 controlling the larger cells; the latter are motoneurones and cause rapid 

 contraction of the heart muscle (52a, 64a, 86, 87). 



There is suggestive evidence for local reflex pathways in the arthropod 

 heart. In the heart of Limulus the frequency and amplitude of beat are 

 influenced by the condition of the heart muscle, but this effect depends on 

 the presence of the cardiac ganglion. In the cardiac ganglia of Crustacea 

 it has been observed that the pacemaker and motor neurones bear dendri- 

 tic processes, which make contact with the cardiac muscle fibres: these 

 processes, it has been suggested, are afferent in function (75). 



The intrinsic cardiac ganglia of crustaceans and Limulus receive regula- 

 tory nerves from the c.n.s. Both inhibitory and acceleratory fibres have 

 been distinguished. In Crustacea the extrinsic cardiac nerves originate 

 from suboesophageal and thoracic ganglia. In Limulus there are inhibitory 

 fibres which arise from the brain, and acceleratory fibres which are believed 

 to originate in abdominal ganglia. Stimulation of the cerebral ganglia 

 inhibits the heart of Limulus, and stimulation of the inhibitory fibres 

 reduces the output of impulses from the cardiac ganglion. 



In decapod Crustacea, acceleratory and inhibitory fibres have been dis- 

 tinguished by the effects of transection and electrical stimulation (Fig. 

 3.19). The extrinsic cardiac nerves of the spiny lobster Panulirus consist of 

 two pairs of cardio-accelerators and one pair of cardio-inhibitors. Stimula- 

 tion of the cardio-inhibitory nerves causes a reduction in amplitude and 

 frequency of heart beat. Effective limits of stimulation lie between 15 and 

 90 shocks per sec, and between these limits the rate and amplitude of beat 

 can be graded by varying the frequency of stimuli. Conversely, stimulation 

 of the cardio-accelerators increases the frequency and amplitude of heart 

 beat (effective stimulation frequencies lie between 5 and 60 shocks per 

 sec). The extrinsic cardiac nerves appear to achieve their effects by depress- 

 ing or enhancing excitabilities of ganglionic pacemaker cells (75, 86, 122). 



In the pericardial cavity of Malacostraca (decapods, stomatopods, 

 isopods) there are peculiar ramifying networks of nerve fibres forming a 

 neuropile termed the pericardial organs (Fig. 3.20). It has been suggested 

 that these have a neurosecretory function. Extracts of the pericardial 

 organs produce marked changes in heart action: their effect is usually 

 excitatory and resembles that produced by adrenaline and noradrenaline 

 (2, 5, 73a). 



The hearts of higher crustaceans (amphipods, isopods, decapods) and 

 Limulus are accelerated by acetylcholine, and the excitatory action of 

 acetylcholine is enhanced by eserine (Fig. 3.21). Pilocarpine and eserine 

 resemble acetylcholine in accelerating the heart. Atropine, which often 

 antagonizes the action of acetylcholine, slows the heart or blocks the effect 

 of acetylcholine in crustaceans, but has a stimulatory effect on Limulus 

 heart. Nicotine initially accelerates, followed by a paralysing effect (Fig. 

 3.21). Adrenaline also has an acceleratory effect on the hearts of crusta- 



