12 



310I.LUSCA. 



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 

 p.-dr 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; s g , 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 Phylogenie 

 der Mollusken." Leipzig, 1877. 



