NERVOUS SYSTEM. INVKRTEBRATA. 49 



D. 48. The nervous system of an Apple- 



isolated excepting the pedal nerves of the right side. The 

 nervous system of this Prosobranch affords a striking 

 example of a condition of the visceral loop, known as 

 zygoneury ; by this is meant the presence of certain con- 

 nections between the sub- and supra-intestinal ganglia and 

 the pleural ganglion of the same side, whereby a kind of 

 false orthoneury arises that to a greater or less extent 

 masks the original streptoneurous condition of the loop. 

 These connections occur commonly among Prosobranchs, 

 and can be present on one or both sides constituting right, 

 loft, or double zygoneury according as the nervous union 

 takes place between the right pleural and subintestinal 

 ganglia, the left pleural and supra-intestinal, or both. In 

 this specimen double zygoneury is shown in an extreme 

 form. The visceral loop passes from the left pleural 

 ganglion beneath the intestine to a small subintestinal 

 ganglion partially fused to the right pleural ganglion this 

 fusion constitutes the right zygoneurous connection. From 

 the subintestinal ganglion the visceral loop runs round the 

 margin of the mantle-cavity as usual, giving off a nerve to 

 the mantle on the right side and several to the viscera from 

 a bilobed abdominal ganglion situated at its posterior ex- 

 tremity. The left arm of the loop, after giving oft' a few 

 small nerves, enters an elongated supra-intestinal ganglion, 

 which gives origin to a stout nerve for the osphradium 

 and several smaller branches that are said to pass across 

 the roof of the mantle-cavity to the gill, which in this 

 mollusc is displaced from the left to the right side by the 

 development of the lung-sac. The supra-intestinal ganglion 

 is connected by a large trunk to the left pleural ganglion 

 (the left zygoneurous connection) and to the right pleural 

 ganglion by a delicate filament that passes dorsal to the 

 intestine and subintestinal ganglion, and v represents the 

 supra-intestinal arm of the streptoneurous loop in a very 

 much reduced condition. 



The pleural ganglia are fused to the pedal to form a pair 

 of suboesophageal masses, in each of which the constituent 

 parts are separated by a shallow depression. They are 

 connected by three commissures one between the pleural 



VOL. II. E 



