. Éléments de l'Anatomie Comparée. 350 co^IPARATIVE ANATOMIE. sont également trouvés dans le Ceplialopoda tliey ; mais sont plus étroitement rapprochées en conséquence de la réduction de leur com- missures. Former un anneau autour de l'oesophage. Dans le Dibranchiata cet anneau est fermé par le cartilage céphalique de telle manière que les nerfs périphériques passent à travers foramina dans elle. Dans Nautilus, la partie supérieure de l'œsophage est formé par un ti'ansversely éliminés masse nerveux (Fig. 186, aa) ; les nerfs des organes sensoriels sont émis à l'autre. Il correspond à la Cerebral gangli

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. Elements of Comparative Anatomy. 350 CO^IPARATIVE ANATOMY. are also found in the Ceplialopoda; but tliey are more closely approximated in consequence of the shortening of their com- missures. They form a ring around the oesophagus. In the Dibranchiata this ring is enclosed by the cephalic cartilage in such a way that the peripheral nerves pass out through foramina in it. In Nautilus, the upper part of the oesophageal ring is formed by a ti'ansversely disposed nervous mass (Fig. 186, aa); the nerves of the higher sensory organs are given off at the sides. It corresponds to the cerebral ganglia, but these also extend some distance ventraUy (hh), and give off, in that region, nerves which pass to the tentacles (W). The ventral segment only of this ring is homologous with the pedal ganglion, as it gives off the nerves for the funnel. A second ventral mass (ec) is attached to the lower nervous mass, which, as we have already said, is partly formed from the cerebral ganglia; this corresponds to the visceral ganglia, and gives off nerves for the mantle (m), in addition to two small trunks which accompany the vena cava, and pass to the gills and vascular system. Each of these two nerves forms a ganglion (d), which, again, supplies the generative appa- ratus. In the Dibranchiata the nervous system is much more concentrated. The cerebral ganglionic masses are placed much more to the sides and ventral surface, so that they unite in the middle line in front of the part which corresponds to the pedal ganglia. The visceral are completely fused with the pedal ganglia, and the only rem- nant of the primitive independence of these ganglia is a narrow point through which the pedal artery passes; the separate character of these ganglia is much more distinct in the Tetrabranchiata. That portion of the cerebral ganglionic masses which still remains above the oeso- phagus is differentiated into several portions. The part which has reached the ventral surface gives off the nerves for th

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