Mollusca. 



289 



four) ; they clearly correspond with the segmental organs of the 

 Chaetopoda.''*' 



The nervous system is peculiar; the typical arrangement is 

 as follows (Fig. 237 B) : above the anterior portion of the alimentary 

 canal lies a pair of cerebral ganglia, connected by a com- 

 missure ; from these a nerve cord runs, on either side, round the 

 oesophagus to a pair of pedal ganglia, lying in the foot, 

 which are likewise connected by a commissure ; behind the cerebral 

 ganglia lie a pair of pleural ganglia, joined by one pair of 

 connectives to the cerebral, by another to the pedal ganglia. Prom 



C-fp 



i9 



Fig. 237. Diagram of the central nervous system in variouB Mollusca. 

 A — B Gtastropods, C — D Lamellibrancha. h, cerebral ganglion, / foot, p pleural ganglion, 

 ig visceral ganglion, i visceral loop, n pedal nerve. — Orig. 



each pleural ganglion there arises a nerve cord, which is usually 

 long, and runs posteriorly through the body, uniting with that 

 of the other side to form a loop termed the visceral loop: 

 the visceral ganglia lie upon it, posteriorly. Great differences 

 appear in the various Molluscs j the nerve cords may be long or 

 short, sometimes so short that all the ganglia lie close together 

 and fuse to form a single mass (Fig. 237 D). 



Tte cerebral ganglia apparently correspond with the same structures in the 

 Ohaetopoda, the pedal ganglia with the first ventral pair of the same ; one pair of 

 nerves, the pedal nerves, which proceed from the pedal ganglia and run 

 posteriorly, and, in various MoUuscs {e.g., in Chitons and certain Gastropods, 

 Fig. 237 A), are very thick, and are connected by fine transverse cords, probably 

 correspond with the ventral nei-ve cords of the Ohsetopods. The pleural ganglia 

 and the visceral nerves must be regarded as new structures. 



* In many Mollusca — Cephalopoda, many Lamellibranchs, several Gastropoda — 

 certain parts of the epithelial covering of the pericardium are glandular. Similar 

 epithelium may occur upon evaginations of the auricles, or there may be true 

 glandular evaginations of the pericardial wall. Apparently all these structures, 

 which are called indiscriminately pericardial glands, are excretory organs. 

 The waste substance formed in the cells, partly as concretions, is doubtless got rid of 

 by the kidneys. 



