. Elements of Comparative Anatomy. 344 COMPARATIVE ANATOMY. organs wliicli are placed in the head, prove that these ganglia are homologous with the cerebral ganglia of Vermes (and of Arthropoda). The pedal ganglia may also be derived from a more simple condition, for in many of the lower Mollasca we find them replaced by an arrangement which corresponds to the ventral chord of the Annulata. Longitudinal trunks are given off from the pedal ganglia, and are distributed along the foot; since they are con- nected together by transverse chords, they are arranged in the same manner as a ventral nerv
. Elements of Comparative Anatomy. 344 COMPARATIVE ANATOMY. organs wliicli are placed in the head, prove that these ganglia are homologous with the cerebral ganglia of Vermes (and of Arthropoda). The pedal ganglia may also be derived from a more simple condition, for in many of the lower Mollasca we find them replaced by an arrangement which corresponds to the ventral chord of the Annulata. Longitudinal trunks are given off from the pedal ganglia, and are distributed along the foot; since they are con- nected together by transverse chords, they are arranged in the same manner as a ventral nerve-chain. Although there may be nothing really fresh in this arrangement of the nervous system, inasmuch as the two ventral (or pedal) ganglia must be regarded as a concentrated nervous mass, which is broken up in lower forms, and constitutes a ventral ganglionic chain; yet the greatly-developed visceral ganglia form an arrangement which is nothing like so well marked in the Vermes as it is here. lu the Mollusca the visceral ganglia are not only of importance as forming a part of the general nervous system, but they may also fuse with the cerebral ganglia, owing to the gradual shortening of their commissures. New, and primitively peripherally-placed parts, are thereby added on to these central organs ; and it becomes a matter of doubt whether or no these ganglia, which formerly belonged to the visceral nei'vous system, should still be regarded as belonging to it. These parts of the nervous system which supply the viscera (heart, branchial apparatus, and generative organs) are the cause of great complica- tions of the whole system ; owing to the way in which they vary in number in different divisions, they make comparison very difficult, as indeed also do the great modifica- tions in position undergone by the primitive ganglia, in consequence of the abbreviation or elongation of their commissures. The nervous system of the Placo- phora is one of the lowest found. A nervous band f
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