. Animal physiology. Physiology, Comparative; Physiology, Comparative. 334 NERVOUS SYSTEM OF PECTEN. the sides of the oesophagus; these ganglia are connected with the very sensitive tentacula which guard the mouth; and they evi- dently correspond, both in their position and functions, to the brain of higher animals, whilst the posterior ganglion has for its office to regulate the respiratory movements. In the Oyster and others of the lower Conchifera, which have no foot, these are the only ganglia; but in those which possess a foot, such as the Pecten, we find an additional ganglion connected
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 334 NERVOUS SYSTEM OF PECTEN. the sides of the oesophagus; these ganglia are connected with the very sensitive tentacula which guard the mouth; and they evi- dently correspond, both in their position and functions, to the brain of higher animals, whilst the posterior ganglion has for its office to regulate the respiratory movements. In the Oyster and others of the lower Conchifera, which have no foot, these are the only ganglia; but in those which possess a foot, such as the Pecten, we find an additional ganglion connected with it. In the accompanying figure of the nervous system of the Pecten, it is seen that the cephalic ganglia (or ganglia of the head), a a, are situated beneath the oesopha- gus, e; but are connected with each other by a band that arches over it. They are also connected with the pedal ganglion (or gan- glion of the foot) c, and with the branchial or respiratory ganglion, b ; but these last are not connected with each other. In fact, it is found, by careful dissection, that part of the nervous trunks, which pass from the ganglion c into the foot, and from the ganglion b to the respiratory apparatus, originate in the cephalic ganglia; whilst the greater part are connected only with the pedal and branchial ganglia respectively. And there is good reason to believe that, whilst the cephalic ganglia alone are the instruments of sensation and of voluntary power,—so that they exert a gene- ral control and direction over the movements of the animal,—the pedal and branchial ganglia minister to the reflex actions (§. 433) of the organs which they supply. 438. A similar arrangement is found in the higher Mollusca, among which the ganglia are more numerous, in consequence of the greater variety of functions to be performed. Of this we have an example in the Aplysia, a sort of sea-slug, somewhat resem-. Fig. 171.—Nervous System of Pecten. A A, cephalic ganglia ; B, branchial ganglion
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