12 



MOLLUSCA. 



Sr 



lobes, each of which secretes a single shell ; the two valves so formed 

 are united on the dorsal surface by a ligament. 



The internal organization of the Mollusca 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* (figs. 495, 496, 497) consists of a dorsal 

 pair of ganglia lying on the esophagus (only exceptionally 

 fig. 495 resolved into a general ganglionic investment of the com- 

 missure), the cerebral ganglia (figs. 

 496, 497, Cg), from which pass off 

 the sense nerves and an oesopha- 

 geal ring, composed of several 

 fibrous cords. The latter primi- 

 tively gives off two pairs of nerve- 

 trunks. The nerves of the upper 

 and lateral pair are the pallial 

 nerves (fig. 495, PaSt) ; 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, PeSt); 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 

 at the origin of the pedal nerves ; 

 these are the pedal ganglia (figs. 



496, 497, Pg). In 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, " Vergleichende Anatomie des NervenystemsgoenPhl uns yied 

 der Mollusken." Leipzig, 1877. 



PaSt 



Fia. 495. Nervous system of Chiton (after 

 B. Haller). Sr, cesophageal ring; Eg, 

 buccal ganglion ; PeSt, pedal nerve ; 

 PaSt, pallial nerve ; Br, gills. 



