CHAPTER IV. 



THE NERVOUS SYSTEM OF MOLLUSKS. 



For (Several reasons it will be advantageous to depart 

 from the usual zoological order, and consider first the 

 disposition of the nervous system in some of the principal 

 types of the sub-kingdom Mollusca. 



These are animals mostly aquatic and wholly devoid of 

 hollow, articulated, locomotor appendages. Their organs 

 of vegetative life attain a disproportionate development, 

 as may be imagined from the fact that some of the 

 simplest representatives of the class consist of mere 

 motionless sacs or bags, containing organs of digestion, 

 respiration, circulation, and generation. The most complex 

 Mullusks, however, are active predatory creatures, endowed 

 with remaikable and varied powers of locomotion, and with 

 sense organs which are both keen and highly developed. 

 The simpler forms are represented by the motionless 

 Ascidian, and the higher by the active and highly endowed 

 Cuttle-fish. 



It should be mentioned, however, that the tendency of 

 several recent investigations has been to separate the 

 class to which the Ascidians belong altogether from the 

 Mollusca, and to place them as an independent group, 

 having affinities to the lowest Vertebrates. 



The solitary Ascidians may be taken as the typo of 



