THE FORMATION OF THE ORGANS THE NERVOUS SYSTEM. 189 



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

 formed 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 off 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, one and the same ganglion sometimes bearing several different 



SS. 



FIG. 85. A-C, three stages in the development of Fissurdla showing the changes in 

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

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

 snout ; sp, apex of the shell ; .sx, 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 

 system 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 



