The oesophageal orifice is small, and guarded by a strong crescentic or horseshoe-like band of muscular fibres, supposed to be the cause of the difficulty of vomiting in the horse.
Each of these contains a single blindly a, Oesophageal (ventral) mesentery.
As regards their innervation an apparent exception is found in the case of A pus, where the nerves to the antennules arise, behind the brain, from the oesophageal commissures, but this is, no doubt, a secondary condition, and the nerve-fibres have been traced forwards to centres within the brain.
The central nervous system is constructed on the same general plan as in the other Arthropoda, consisting of a supra-oesophageal ganglionic mass or brain, united by circumoesophageal connectives with a double ventral chain of segmentally arranged ganglia.
The brain, or supra-oesophageal ganglion, shows various degrees of complexity.