120 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



and the term connective for a nerve-cord connecting ganglia of a 

 different name. Both kinds are here exemplified, but most of the 

 nerve-cords are connectives, not commissures. On each side of the 

 mouth, just beneath the skin, a cerebro-pleural ganglion is situated, 

 and the dorsal part of the nerve-ring constitutes a short com- 

 missure above the mouth connecting these two ganglia. A pair 

 of pedal ganglia lie close together at the junction of the foot and 

 the visceral mass, resting upon the muscle of the former nearer 

 its anterior than its posterior end. Each side of the nerve-ring 

 may be regarded as a cerebro-pedal connective, since it unites a 

 cerebro-pleural ganglion with the pedal ganglion of the same side. 

 The visceral loop runs back along the dorsal side from the cerebro- 

 pleural ganglia to the under side of the posterior adductor, where 

 it thickens into a closely apposed pair of visceral (parieto-splanchnic) 

 ganglia. Each side of the loop constitutes a very long cerebro- 

 visceral connective running back from one cerebro-pleural ganglion 

 to the visceral ganglion of the same side, to reach which it first 

 traverses the digestive glands and then takes a course along the 

 inner side of the kidney. 



The cerebro-pleural ganglia supply the lips, labial palps, anterior 

 adductor, and probably the otocysts. They also give off anterior 

 pallial nerves which break up into a network in the margin of the 

 front part of the mantle. The pedal ganglia supply the muscular 

 tissue of the foot, and from the visceral ganglia three chief pairs 

 of nerves run out to the gills and mantle. These are (1) in front, 

 branchial nerves, then (2) lateral pallial nerves, and (3) posterior 

 pallial nerves, which supply the tentacles of the inhalent aperture, 

 and break up to form a complex nervous network in the margin 

 of the hinder part of the mantle, continuous with the similar 

 network arising from the anterior pallial nerves. The visceral 

 ganglia also send nerves to the posterior adductor and the rectum. 



A visceral or gastric nerve runs to the digestive glarid and 

 stomach from each cerebro-visceral connective, not far from its 

 anterior end. 



As usual, ganglion-cells and nerve-fibres are the essential parts of 

 the nervous system, the former being entirely confined to the 

 outer part of the ganglia and nerve-origins. 



10. Sense Organs. (1) Tactile Organs. The labial palps act 

 as feelers, and their epithelium comes into close relation with 

 fibres from the cerebral ganglia. The tentacles surrounding the 



