76 



MOLLUSCOUS TYPE OP NERVOUS SYSTEM. 



sense, and evidently having a controlling power over the whole 

 system. In the Molluscous animal, again, as there is no repeti- 

 tion in the several parts of the body, so there is no repetition of 

 ganglia, except on the two sides, when there are corresponding 

 organs to be supplied. Thus we have usually a single or double 

 ganglion in the head (where it exists), or in the neighbourhood 

 of the mouth, connected with the organs of special sense ; another 

 for the actions of the respiratory apparatus, and another for the 

 foot. Of these, the arrangement in the lower forms of Mollusca 

 is represented in Fig. 35, and in the higher, in Fig. 36. 

 There is a total absence of all regularity in the arrange- 

 ment of the several ganglia ; for their position varies con- 

 tinually in the different species according to that of the organs 

 they supply : thus, in the shell- 

 less species, the place of the 

 gills is continually varying ; and 

 that of the respiratory gan- 

 glion varies accordingly, being 



FIG. 35. NERVOUS SYSTEM OF PECTBN. 



A A, cephalic ganglia ; B, branchial 

 ganglion ; C, pedal ganglion ; e, oeso- 

 phagus. 



FIG. 36 KBRVOUS SYSTKM OF 



APLYBIA. 



A, cephalic ganglion ; B, respi- 

 ratory ganglion ; C C, ganglia of 

 the foot and mantle ; D, pharyn- 

 geal ganglion. 



sometimes in the front of the body, and sometimes at its hinder- 

 part, as in the Aplysia. Hence, if we were to find an animal 

 having a series of nervous centres or ganglia scattered through 

 the body not forming a regular series arranged round the 



