18. The Morphology of the Central Nervous System of the Cteno-glossa (Cephalopoda).

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Summary.

The features of the central nervous system of the Ctenoglossa may thus be summarised. The suboesophageal complex shows distinct abdominal, pedal, and brachial elements, not as separate ganglia, but as clearly differentiated parts. This is a condition not found in any Heteroglossate nor in Vitreledonella. The lateral commissure is longer than in Octopus, Eledone, and Vitreledonella, but not so long as in Argonauta. There is a well-developed brachial commissure, as in most of the Octopoda. The degree of fusion between the cerebral and the superior buccal ganglia is slight, but they are in close contact. None of the Incirrata appear to differ much in this respect. The cerebral ganglion is divided into three lobes only (instead of four, as in the Heteroglossa), and the verticalis is not subdivided into lateral and median gyri. In Vitreledonella there is one lobe less and the posterior is tripartite (?= subdivisions of the verticalis).

It will thus be seen that as regards the brachial ganglion the Ctenoglossa are less specialized than the Incirrata (including Vitreledonella). As regards the length of the lateral commissure the same is true, though Argonauta is even less specialised. They agree with the other Incirrata in possessing a well-developed brachial commissure and in the close approximation of the cerebral and superior buccal ganglia. The cerebral ganglion is less subdivided than in the Heteroglossa.

The result of this study supports the view recently put forward that the Ctenoglossa and Vitreledonellidae should be separated, as their central nervous systems are definitely dissimilar. Whether on the characters of the nervous system alone one would treat the latter as more closely related to the Incirrata I am somewhat doubtful. When I previously examined this question I was impressed by the great degree of condensation in the system of Vitreledonella and its likeness therein to the other Heteroglossa. It is now, however, apparent that the system contains special features, and though it might pass for a more condensed type of, e. g., Octopod system, it is less evolved in the character of the cerebral ganglion. On this system it seems to me an open question whether we should regard Vitreledonella as a highly evolved Heteroglossate in which the cerebral ganglia have undergone less division, or whether it should be placed in a group of its own equivalent to the Heteroglossa.

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