THE FORMATION OF THE ORGANS— THE NERVOUS SYSTEM. 



189 



researches, the cerebral ganglion or part of it, is, in certain cases, 

 funned by an invagination of the ectoderm. So far as is as yet 

 known, the cerebral ganglion alone has such an origin; all the other 

 ganglia arise as ectodermal thickenings which later split oft' from 

 this germ-layer. 



An accurate knowledge of the structure of the nervous system of the adult 

 is very desirable as a help to understanding the processes of development, 

 especially as some confusion prevails as to the naming of the different parts 

 of that system, oue and the same ganglion sometimes bearing several different 



FIG. 85. — A-C, three stages in tin- development of Fisswrella .showing the changes in 

 the shell (after BOTJTAN). The animal, as depicted in C, has very nearly attained 

 the adult form. /, part of the foot; ma, mantle; ms, mantle-slit; s, shell; sn, 

 snout ; sji. apex of the shell; ss, shell-cleft ; t, tentacles. 



names, while, on the other hand, different ganglia receive similar names. 

 We shall therefore describe side by side some of the principal types of nervous 

 -\stem found in the Gastropoda (Fig. 86 A-C). 



The nervous system of the Gastropoda consists of the two cerebral ganglia, 

 which are connected by the supra-oesophageal cerebral commissure {A-C, eg). 

 Below the oesophagus, and connected with the cerebral ganglia by connectives, 

 lie the pedal ganglia (peg), which innervate the foot and are joined together 

 by a commissure. In this way a ring corresponding to the oesophageal ring 

 of the Annelida and Arthropoda is formed. The resemblance ceases when we 



