1. The three systems of nerves, viz. the local system, the regulator nerves, and the nerves of the arterial valves, which were previously described by the writer as innervating the heart of the Decapod Crustacea, have also been found in Squilla mantis.
2. The local system consists of not less than fourteen neurons. Their cells are situated in a nerve-trunk running alongside the dorsal surface of the heart, and, with the exception of the three anterior elements, lie at regular intervals each behind a pair of the ostial orifices. The cells give off the following processes: (a) the axons which form the chief part of the fibres in the ganglionic trunk and which after sending off many branches end on the muscle-fibres of the myocardium; (b) the dendrites--short arborescent branches arising both from cell-bodies and axons, and ending in the neighbourhood of the trunk on themuscle-fibres too; (c) short collaterals ending in fine networks of fibrils in the ganglionic trunk.
3. The system of regulator nerves connecting the local system with the central nervous system, in the Decapoda consisting of one pair of nerves, is represented in the Stomatopoda by three paired nerves which in our description have been termed Nervi cardiaci dorsales. For the designation of each of them the letters α,β, and γ have been used. Their course indicates that they originate in the large thoracic ganglionic mass. After passing on the dorsal side of the extensor muscles these nerves approach the heart from its dorsal side, and enter its ganglionic trunk in the region of the fourth body-segment. The nerve a is made up of one thick fibre only, the nerves β and γ contain one thick and several thinner fibres each.
In the ganglionic trunk two sets of fibres given off by the dorsal nerves can be distinguished: one of them, termed System I, is made up of thicker fibres whose branches give synapses with the cells, collaterals, and dendrites of the local neurons; the other, called System II, consists of thinner fibres accompanying the long branches of the axons which pass to the muscles.
4. The system of nerves supplying the arterial valves is made up of (a) the anterior cardiac nerve running to the valve of the anterior aorta; and (b) the segmental nerves of the heart passing in each metamere to the valves of the paired arteries. There are, in all, fifteen pairs of these nerves. The last pair supplies the valves of the fifteenth pair of arteries and the valve of the posterior aorta. Each segmental nerve sends off anastomotic branches to the contralateral nerve, but does not show any connexions with the nerves of the neighbouring segments. In this respect these nerves in Squilla differ from those in the Decapods since in the latter they are all interconnected by anastomosing fibres. On the other hand, in Squilla as well as in Decapods the anterior cardiac nerve has no connexion with the segmental nerves of the heart.
5. With regard to the function of the nerve-elements enumerated above, the local system is to be considered as an autonomic apparatus which rules the beat of the heart, whereas the dorsal nerves convey the inhibitory and accelerator impulses from the central nervous system. The first of the dorsal nerves, α, has been found carrying the inhibitory impulses. The stimulation of the two following nerves, β and γ, quickens the beat of the heart, but this effect of the physiological experiment does not exclude the possibility that the nerves β and γ contain both inhibitory and accelerator fibres.
The two sets of fibres in the ganglionic trunk which have been termed Systems I and II are probably concerned the former with the inhibitory and the latter with the accelerator action.
The function of the nerves of the arterial valves probably consists in the maintaining of a tonic contraction of the muscles of the valves.
Innervation of an amphipod heart