. Elements of comparative anatomy. Anatomy, Comparative. NEKVOUS SYSTEM OE MOLLUSCA. 349 tlie cerebral (Fig. 185); the visceral ganglia also get to be connected with these latter owing to the abbreviation and complete disap- pearance of the commissures between them (zEolidia). A dorsal plexus of ganglia, which lies on the pharynx, is formed by the more intimate fusion of these ganglia; in each half of this plexus the various ganglia which compose it can be more or less distinctly made out; several commissures are given off, which surround the pharynx (Doris, Tritonia). This connection of the g
. Elements of comparative anatomy. Anatomy, Comparative. NEKVOUS SYSTEM OE MOLLUSCA. 349 tlie cerebral (Fig. 185); the visceral ganglia also get to be connected with these latter owing to the abbreviation and complete disap- pearance of the commissures between them (zEolidia). A dorsal plexus of ganglia, which lies on the pharynx, is formed by the more intimate fusion of these ganglia; in each half of this plexus the various ganglia which compose it can be more or less distinctly made out; several commissures are given off, which surround the pharynx (Doris, Tritonia). This connection of the ganglia of either side may lead at last to the complete fusion of the ganglionic masses of either side into a single one, and in correlation with this fusion the previously separate commissures may be represented by one only (Tethys). This appears to be a lower stage, but it must not be regarded as an eai'ly arrangement, but as the final one in a series of diffe- rentiations, exactly similar to what we found to obtain in the Arthropoda. In the same way as the nerves, which pass off from the single nervous mass, indicate that it is made up of separate portions, the circum-oesophageal com- missures prove that the ventral ganglia have become more dorsal in position. The nervous system of the Branchi- opneusta is similar in many points to that of the Tectibranchiata, and similar relations can also be made out in the Nephropneusta. Both divisions are characterised by the development of their cerebral ganglia into several por- tions, which can be distinguished as such even from the § 269. Fig. 185. Central nervous system of one of the JE olid ire (Fiona atlantica). A Supra-cesophageal mass, formed by the anterior cere- bral, and posterior visceral or branchial ganglia. B Pedal gan- glia. C Buccal ganglia. D Gastro- oesophageal ganglia. a Nerve to the superior (hinder) tentacles. b Nerve to the inferior tentacles, c Nerve to the generative organs. d Pedal nerves, e Commissu
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