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Show 674 . MR. J. GRAHAM KERR ON THE [June 18, to both these statements, Professor Lankester seems to be iu error, as a complicated buccal system was described and figured by H . v. JheringL, while at least part of an enteric system was described by Keferstein2. In regard to the buccal nervous system it seems advisable to give a short account, however, as von Jhering's remarks are very brief, while in the construction of his diagram a curious blunder seems to have been made, which has been perpetuated by its being copied by leading text-books. In effect, what he figures as the cerebro-pharyngeal connective is really the forward prolongation of the pharyngeal ganglion, which, uniting with its fellow in the middle line, forms the anterior infra-buccal commissure. On the other hand, the two nerves figured as emerging from the pharyngeal ganglia laterally are the cerebro-pharyngeal connectives, of which there are not one, but two, on each side. In fact, by rotating the portion of his figure representing the buccal nervous apparatus through 180° about an axis passing through the pharyngeal ganglia and joining up the cut ends as indicated above, one gets quite an accurate diagram. N o doubt v. Jhering's slip arose through dissecting and figuring this part of the nervous system after the buccal mass had been removed from its connection with the animal. The cerebro-pharyngeal connectives are two thick nerve-trunks on each side, taking their origin from the adoral border of the supra-cesophageal nervous mass. Enclosed in a dense sheath of connective tissue, they pass to the sides of the buccal mass. For the first part of their extent their course is highly sinuous, a character probably to be connected with the protrusibility of the buccal mass and the consequent very variable distance between it and the fixed circuin-cesophageal parts of the nervous system. Pursuing a slightly convergent course the two connectives reach the lateral aspect of the buccal mass, and there unite in the pharyngeal ganglion (fig. 4, ph.g.)-a triangular structure with its apex directed towards the mouth-and another of its angles external. It lies on the muscles of the buccal mass immediately beneath the skin. The two pharyngeal ganglia are connected to one another by a longer anterior and a shorter posterior commissure, on the course of the latter being the slightly s wollen " buccal ganglia." The anterior commissure (fig. 4, ant.com.), figured by v. Jhering as cerebro-pharyngeal connective, is a flattened band nearly 1 millimetre in breadth, and pursuing a O-shaped course immediately beneath the skin, and just within and parallel to the margin of the lower mandible. As the ganglion tapers off into the commissure, it gives off numerous small and several larger filaments to the skin-fold surrounding the mandibles. From the adoral part of the com-missure also some very fine strands pass off to the same structures. The internal angle of the pharyngeal ganglion is prolonged towards the middle line into the posterior commissure, which soon 1 Vergl. Anat, des Nervensystems der Mollusken, p. 263. 2 Bronn's ' Thier-Reich,' Malacozoa, p. 1373. |