The supra-oesophageal ganglion of Nephtys varies considerably in its size and gross morphology from one species to another. The ganglion is relatively larger in small species and then extends into the anterior body segments. It is epidermal and in contact with the cuticle in the prostomium, sub-epidermal in the anterior segments. In a number of species a pair of conspicuous posterior lobes to the ganglion has been developed. There is a tendency, particularly in the smaller worms, for the ganglion to be bifurcated at its posterior, and sometimes also at its anterior margin. The sensory nerves are as follows: tegumentary nerves, extremely variable in number and disposition, arising from the front of the ganglion; two pairs of antennary nerves arising immediately above the roots of the circum-oesophageal connectives, but generally distinct from them; and a pair of anterior optic ‘nerves’ supplying the paired, single-Celled photoreceptors. The only other nerves arising from the ganglion are the nuchal nerves which arise from it opposite the nuchal organs, whatever the position of the brain. Since the latter may barely protrude into the prostomium, these nerves emerge from the anterior part of the brain in some species, instead of near the posterior end of the ganglion. The circum-oesophageal connectives are extremely long because the sub-oesophageal ganglion lies in the fifth segment. In their passage through the lateral lips of the mouth and along the sides of the gular membrane, they give off three pairs of stomatogastric nerves and the segmental nerves of the first two segments. The third and fourth segmental nerves arise from the front of the sub-oesophageal ganglion, those of the fifth segment from the back of the ganglion. The stomatogastric nervous system is also described. The entire nervous system shows little specialization compared with that of most other errant polychaetes.

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