. Elements of Comparative Anatomy. 31U COMPARATIVE ANATOMY. of masses of ganglia, which lie uear the oesophagus (Fig. 165-, d). A larger gaugliou lies transversely across and below the oesophagus, or rather, owing to the downward bend of the a3sophagus, behind it (in the Terebratulida). Two nerve-trunks pass off from it towards the posterior region, and break up into nerves for the stalk; they are pro- vided with swellings {ii'). The nerves for the ventral lamella of the mantle are given off from the swellings on these trunks. From the large ganglion, however, a nerve- trunk is given off, on e
. Elements of Comparative Anatomy. 31U COMPARATIVE ANATOMY. of masses of ganglia, which lie uear the oesophagus (Fig. 165-, d). A larger gaugliou lies transversely across and below the oesophagus, or rather, owing to the downward bend of the a3sophagus, behind it (in the Terebratulida). Two nerve-trunks pass off from it towards the posterior region, and break up into nerves for the stalk; they are pro- vided with swellings {ii'). The nerves for the ventral lamella of the mantle are given off from the swellings on these trunks. From the large ganglion, however, a nerve- trunk is given off, on each side, to the dorsal lamella of the mantle, as well as a nerve to the arms. Two fine filaments surround the oesophagus and pass into a small ganglion, placed in front of, and therefore on the dorsal side of, the oeso- phagus; this is connected with the other ganglion by a commissure. In this way an oesophageal ring is formed, and the only question is whether the small upper ganglia represent central ganglia or not. If so, then there is this peculiarity, that the nerves for the arms arise from the ventral gan- glia, and we can hardly re- gard the arms themselves as homologous with the ten- tacles of Vermes, even if it could be shown tliat the gan- glionic parts in the oesopha- geal ring had altered their position. The ventral gan- glionic mass must appa- rently be compared with a shortened ventral gangli- onic chain; but to make a safe comparison we must have more exact information as to the facts. The slight development of the superior ganglia is correlated with the absence of higher sensory organs, and this absence appears to be an acquired condition, for the four pigment spots, found on the first segment in the larva, point to the existence of visual organs (Fig. 163), and lead us to suppose that there were eyes in the ancestral forms. The two vesicles found in another larval form similarly point to the previous existence of auditory Fig. 165. Nervous system of Wa
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