Structurt . 



I I 



Although there is no brain in the si rid sense of the word there 

 is sometimes a concentral ion of ganglia at the anterior end w hich 



may become so intimate as to constitute a solid mass. 



A considerable variety of sense-organs is present, of which we 

 can only mention the principal ones. 



Eyes, of greater or less complexity, are developed, most often 

 in the region of the head, but sometimes on the mantle. The 



Central nervous system of Patella vulgata, dorsal aspect. 



I. tentacular nerve ; II. cerebral ganglion ; in. cerebro-pedal connective ; 

 iv. cerebro-pleural connective ; vni. pedal ganglion ; ix. visceral commissure ; 

 XI. abdominal ganglion ; xn. pallial ganglion ; xiv. stomatogastric ganglion ; 

 \\ ii. cerebral commissure. 



From Lankester's Treatise on Zoology (by permission of Messrs A. & C. Black). 



remarkable eyes of some Cephalopoda rival those of Vertebrates 

 in the elaboration of their structure. 



Organs of smell (osphradia, rhinophores) are found on the 

 head or, in aquatic forms, at the entrance to the mantle-cavity, 

 near the gills. 



Otocysts are found in most molluscs in the tissues of the foot. 

 They may, as was formerly believed, be organs of hearing, but 

 they arc more probably connected with the sense of balance and 

 direction. 



