. Elementary text-book of zoology. Zoology. 12 lobes, each of which secretes a single shell; the two valves so formed are united 011 the dorsal surface by a ligament. The internal organization of the Mbllusca presents as many differences as does the external form. Like the external form, the internal structure also frequently presents surprising devia- tions from the bilateral arrangement. The nervous system:;: (tigs. 495, 496, 497) consists of a dorsal of ganglia lying on the oesophagus (only exceptionally— tig. 495—dissolved into a general gangiionic investment of the com-
. Elementary text-book of zoology. Zoology. 12 lobes, each of which secretes a single shell; the two valves so formed are united 011 the dorsal surface by a ligament. The internal organization of the Mbllusca presents as many differences as does the external form. Like the external form, the internal structure also frequently presents surprising devia- tions from the bilateral arrangement. The nervous system:;: (tigs. 495, 496, 497) consists of a dorsal of ganglia lying on the oesophagus (only exceptionally— tig. 495—dissolved into a general gangiionic investment of the com- missure), the cerebral ganglia (figs. 496, 497, C,j), from which pass off the sense nerves and an oesopha-. geal ring, composed of several PaSt fibrous cords. The latter primi- tively gives off two pairs of nerve- truiiks. The nerves of the upper and lateral pair are the pallial nerves (fig. 495, Pa$t); they supply the lateral parts of the body and the mantle. The nerves of the ventral pair are placed nearer the middle line, and are known as the pedal nerves (fig. 495, Pe/St); they are connected together by transverse commissures (fig. 495) and innervate the muscles of the foot. This arrangement,, found in the simplest form in. Chiton, agrees essentially with that of the Gephyrean-like genus, Neomenia. At a more advanced stage, two large swellings are found FIG. -195.—Nervous system of Chiton (after . B. nailer). Sr, oesophageal ring; sg, at the origin of the pedal nerves j buccal ganglion; P< st, pedal nerve; these are the pedal ganylia (figs. PaSt, pallial nerve; Br, gills. „_ T1 . T , .. 496, 497, Pg). Ln addition, a third group of ganglia, known as the visceral ganglia, is also found. The arrangement of the latter ganglia is very various ; they are some- times fused with the cerebral, sometimes with the pedal ganglia, and are sometimes broken up into several groups of ganglia. They are * H. v. Jhering, " Vergleickende Anatomie des Nervensystems and Phylogeni
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectzoology, bookyear1884
