. Elements of comparative anatomy. Anatomy, Comparative. 350 COMPARATIVE ANATOMY. wiw are also found in the Cephalopoda; but they 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 transversely disposed nervous mass (Fig. 186, aa); the nerves of the higher sensory organs are given off at the sides. It corresponds
. Elements of comparative anatomy. Anatomy, Comparative. 350 COMPARATIVE ANATOMY. wiw are also found in the Cephalopoda; but they 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 transversely 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 ventrally (bb), and give off, in that region, nerves which pass to the tentacles (ft). 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 (cc) 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 giv
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectanatomycomparative
