292 



MOLLUSCA 



[CH. 



10 



or sometimes the brain (1, Fig. 132) ; but there is no reason to think 

 that they are any more important to the animal than the others. 

 Underneath the oesophagus there is what at first sight seems to 

 be a compact nervous mass, connected with the supra-oesophageal 

 ganglia by a commissure on each side forming a nerve collar 

 (Fig. 132). Closer inspection shows that this mass is -perforated 



by a hole through which passes the 

 great anterior artery from the ventricle, 

 and that from both the lower and 

 upper halves a separate nerve comes 

 off to go to the cerebral ganglia. Thus 

 the apparently simple nerve collar con- 

 sists of two commissures on each side 

 united in a common sheath. Between 

 them a minute nerve passes down, to 

 end finally in a minute membranous 

 sac lined by ciliated cells and cells 

 with sense hairs and containing fluid 

 in which a little ball of carbonate of 

 lime floats. This sac, the otocyst, is 

 the only other important sense-organ, 

 besides the eyes, which the snail pos- 

 sesses. It is difficult to dissect, but 

 if the small bivalve Cyclas be taken, 



1. Cerebral ganglion 2. Pedal, t h e s h e n opened and the foot cut off 

 pleural, and visceral ganglia. r 



3. Bucoal ganglia. 4. Nerve and slightly compressed, or if one of 



to lips 5. Olfactory nerve, the transparent Molluscs, such as 



6. Optic nerve. 7. Pleuro- , . . n 



cerebral commissure. 8. Pe- Pterotrachea, which float on the sur- 



do-cerebral commissure. 9. f ace O f the sea, be examined, it is 



Genital nerve. 10. Nerve to - . 



mantle. 11. Nerve to vis- perfectly easy to see both otocysts 



cera< with the microscope. It used to be 



supposed that the function of this organ was to perceive sound, but 

 whilst it is probable that some vibrations of the air affect it, it is 

 nearly certain that, like the otocysts of Medusae and Arthropoda, 

 its main function is to enable the Mollusc to keep its balance by 

 allowing it to perceive whether it is leaning on one side or not. 

 As the snail changes its position the little ball inside rolls about 

 and affects different parts of the wall of the vesicle, and hence 

 probably different fibres in the nerve which supplies it. 



Not all Mollusca possess eyes, but all, except perhaps the 

 Oyster, which never moves, possess otocysts. The experiment of 



Fia. 132. View of nervous sys- 

 tem of Helix pomatia. 



