Nervous system

Nervous system

The circum-oesophageal nerve ring is very compact and positioned a short distance behind the buccal apparatus; it is rather symmetric and characterized by very short commissures ( Fig. 8B). The cerebral ganglia are especially well-developed and situated dorsally on either side of the anterior oesophagus; they possess well-defined labial lobes and are connected by a short, thick commissure. The buccal ganglia are large and rounded, situated on the postero-lateral sides of the buccal mass at the emergence of the anterior oesophagus. The pleural ganglia are large and lie immediately below the cerebral ganglia; they are separated from these by a slight constriction. Ventrally the pleural ganglia are demarcated from the pedal ganglia by another slight constriction. The pedal ganglia are situated within the floor of the haemocoel, below the buccal apparatus. Statocysts bearing numerous statoconia are laterally slightly separated from the pedal ganglia, below the level of the commissure and positioned more dorsal at the left than at right.

The pedal and cerebral ganglia are connected by comparatively long, slender, curving cerebro-pedal connectives. Otherwise, the constrictions between cerebral and pleural ganglia and between pleural and pedal ganglia are limited and these ganglia appear to be fused rather than interconnected with connectives. There is no pleural commissure, nor are there zygoses between the pleural and sub- or supra-oesophageal ganglia. The connectives between pleural and oesophageal ganglia are long. The oesophageal ganglia lie alongside but slightly separated from the mid-oesophagus, roughly at the same antero-posterior position, but the supra-oesophageal ganglion is larger and more elongate than the sub-oesophageal ganglion. Long connectives from the supra-oesophageal ganglia continue posteriorly towards the single, small visceral ganglion. This ganglion overlies the oesophagus at the posterior end of the cephalic haemocoel; it produces a large nerve dorsally, and a smaller nerve at the left side of the oesophagus.