Between the nerves to the jaws and those to the oral papillæ a number of small nerves take their origin. Three of these on each side pass in a dorsal direction and one or two in a ventral one.
The Supra-œsophageal Ganglia.—The supra-œsophageal ganglia (figs. 8 and 22) are large, somewhat oval masses, broader in front than behind, completely fused in the middle, but free at their extremities. Each of them is prolonged anteriorly into an antennary nerve, and is continuous behind with one of the œsophageal commissures. On the ventral surface of each, rather behind the level of the eye, is placed a very peculiar protuberance (fig. 22, d), of which I shall say more in dealing with the histology of the nervous system.
A number of nerves arise from the supra-œsophageal ganglia, mainly from their dorsal surface.
In front are the immense antennary nerves extending along the whole length of each antenna, and giving off numerous lateral twigs to the sense organs. Near the origin of the antennary nerves, and rather on the dorsal surface, there spring a few small twigs, which pass to the skin, and are presumably sensory. The largest of them is shewn in Pl. 50, fig. 19A. About one-third of the way back the two large optic nerves take their origin, also arising laterally, but rather from the dorsal surface (Pl. 50, fig. 19D and E). Each of them joins a large ganglionic mass placed immediately behind the retina. Nearly on a level with the optic nerves and slightly nearer the middle dorsal line a pair of small nerves (fig. 19D) spring from the brain and pass upwards, while nearly in the same line with the optic nerves and a little behind them a larger pair of nerves take their origin.
Behind all these nerves there arises from the line of suture between the two supra-œsophageal ganglia a large median nerve which appears to supply the integument of the dorsal part of the head (Pl. 48, fig. 8; Pl. 49, figs. 11-14, dn).
Sympathetic System.—In addition to the nerves just described there are two very important nerves which arise near the median ventral line, close to the hind end of the supra-œsophageal ganglia. The origin of these two nerves is shewn in the surface view (fig. 22, sy, and in section in fig. 11). They at first tend somewhat forwards and pass into the muscles near the epithelium lining the groove on each side of the tongue. Here they suddenly bend backwards again and follow the grooves into the pharynx.
The two grooves are continuous with the two dorsal angles of the pharynx; and embedded in the muscles of the pharynx, in juxtaposition with the epithelium, these two nerves may easily be traced in sections. They pass backwards the whole length of the pharynx till the latter joins the œsophagus. Here they at once approach and shortly meet in the median dorsal line (fig. 16). They can only be traced for a very short distance beyond their meeting point. These nerves are, without doubt, the homologues of the sympathetic system of Chætopods, occupying as they do the exact position which Semper has shewn to be characteristic of the sympathetic nerves in that group, and arising from an almost identical part of the brain[571].
Histology of the Nervous System.
Ventral Cords.—The histology of the ventral cords and œsophageal commissures is very simple and uniform. They consist of a cord almost wholly formed of nerve-fibres, placed dorsally, and a ventral layer of ganglion cells (figs. 16 and 20).
The fibrous portion of the cord has the usual structure, being formed mainly of longitudinal fibres, each probably being a bundle of fibres of various sizes, enveloped in a sponge-work of connective tissue. The larger bundles of fibres are placed near the inner borders of the cords. In this part of the cord there are placed a very small number of ganglion cells.