The central nervous system ofNautilus

Abstract

The central nervous system of Nautilus shows greater similarity to that of coleoid cephalopods than appears at first sight. In the area where the three main cords of the nervous system meet there is a region comparable in position to the magnocellular lobe of coleoids, and it contains large cells. It receives some static nerve fibres and is the origin of the nerves of the ocular tentacles. The anterior suboesophageal cord is not a single entity. The brachial nerves and nerves of the hood arise from its anterior part, which is directly continuous with the cerebral cord. The funnel nerves arise from a distinct part, continuous with the magnocellular and palliovisceral regions. If the tentacles are innervated from a region derived from the cerebral cord then they cannot be closely compared with the foot of other molluscs. The cerebral cord shows no clear internal division into lobes, but it is nevertheless organized on a plan recognizably like that of coleoids. Its anterior portion contains large cells and gives rise to the connectives that control the buccal mass. It receives the labial nerves and probably gustatory fibres. In the hinder part of the cerebral cord four regions are recognized. An outer dorsal plexiform zone receives afferents from many sources and perhaps serves to allow responses to combinations of inputs. It is especially developed as lateral cerebral lobes at the entry of the brachial nerve fibres. This zone may be compared with the inferior and superior frontal lobes of Octopus. Fibres pass from the plexiform zone through a layer of small cells to a laminated zone of specialized neuropil. This region corresponds approximately to the vertical lobe of coleoids, but the similarity is not very great. The centre of the cerebral cord contains larger cells, probably providing the output channels to other centres. The ventral portion contains commissural bundles. The olfactory lobes are relatively larger and the optic lobes smaller than in coleoids. Both are lateral continuations of the cerebral cord and have the same basic structure as the latter. The optic nerve fibres do not form a chiasma between the retina and the optic lobe. The optic lobe shows a general similarity to that of coleoids but there is no external granular layer and no peduncle lobe. There is no distinct optic gland but cells that perhaps represent optic gland tissue occur between the optic and cerebral lobes. The statocyst is a simple sack with no signs of macula or crista. Its duct remains open in the adult. The static nerve fibres run partly to the magnocellular lobe, partly to the cerebral cord. The plan of the cerebral cord of Nautilus thus appears as a general sketch of the system that exists in coleoids. The ‘higher’ centres for producing responses from combinations of inputs and perhaps for memory storage are only beginning to emerge from an undivided centre for the reflex control of the operations of feeding. The fact that Nautilus has remained macrosmatic and has poor vision may be connected with the relative simplicity of its higher centres. Nevertheless, its nervous system contains vastly more channels and complex parts than are found in any non-cephalopod mollusc.