12 NERVOUS SYSTEM. 



their origin. Almost all of the nerve-cords arising from these 

 three ganglionic pairs may also develop ganglionic enlargements 

 whence numerous nerves in their turn originate. Such is the 

 general plan of the nervous sj^stem in prosobranchiates, differing 

 but little from that of the lamellibranchiates or bivalve moUusca. 



The most important modifications of this arrangement are 

 found in Haliotis, Fissnrella and Chiton. In Haliotis the 

 cerebral ganglia are dispersed, not very distinct, but united by 

 a large commissural Ioojd, whence proceed the nerves of the 

 cephalic tissues and organs. The suboesophageal mass furnishes 

 on either side a very large tnink consisting of two superposed 

 nerves : the pallial and pedal, the latter being united by numerous 

 transverse commissures. The commissures of the visceral 

 ganglia are crossed. 



According to the observations of von Jhering the pedal ganglia 

 of Fisurella are elongated and united by a series of transverse 

 commissures. 



The nervous system of Chiton (v, 68) shows a- simple loop 

 passing over the 03sophageal biilb, and united below by a com- 

 missure. These mollusks have neither eyes nor tentacles and 

 their mouth is reduced to a simple orifice, consequently the 

 cerebral ganglia are atrophied. The pedal nerves are united by 

 transverse commissures, as in Haliotis and Fissnrella and in 

 the annelids. 



In the opisthobranchiates, the ganglia are closely joined, yet 

 distinguishable. The cerebral ganglia are placed above and in 

 front, the visceral below and in the rear, the pedal without ; the 

 two latter connected from side to side by commissural loops. 

 From the cerebral ganglia arise olfactory nerves, which develop 

 a large special ganglion in their course. In Aplysia and Bulla 

 there is a very large genito-branchial ganglion, distant from the 

 suboesophageal ring, with which it is united by a double com- 

 missure. In Tethys the ganglia are fused into a single body, 

 furnished with a commissure forming a nearly complete circle. 



In the pulmonifera or snails (v, 67), the cerebral ganglia are 

 distant in Limax, Yaginula and Oncidium, but join, without 

 commissure in Glandina, and Testacella. Two commissures 

 proceed from each of these ganglia, the first joining the pedal 

 the second the most anterior of the visceral ganglia of its side. 

 Three additional visceral ganglia, with the two pedal ganglia 

 form a suboesophageal ring, united by commissures as in Limnsea 

 or Zonites, or close without connectives as in Glandina and 

 Testacella. Through the midst of the circle passes a branch of 

 the aorta. It will be seen that the visceral ganglia are five in 

 number ; their disposition is usually two on the right and three 

 on the left side in snails having dextral shells and vice versa in 



ose having sinistral shells. These ganglia innervate the 



