PHYLOGEXESIS IX CLASS/FICA T/OX. 249 



from without, or sensation; sensibility as active, or excita- 

 tory of the functions of origans, reflex or motor action. The 

 differentiation of the nervous system corresponds with this 

 distinction. In the lower Mollusca the distinction between 

 the two functions is little seen; but in Gastropoda, for in- 

 stance, and still more in Cephalopoda, special organs are dif- 

 ferentiated for sensation, and the nervous system is in com- 

 munication with each of the differentiated sets of organs, 

 stimulating and directing their activity. All this differentia- 

 tion is associated with the distinction of polarity of motion ; 

 the nervous system is essentially co-ordinative, and binds to- 

 gether the activity of organs in the way of compensating for 

 the separation of parts due to their differentiation and devel- 

 opment in size. The nervous system compensates for sepa- 

 ration of the functional activities of the organism, and the 

 circulatory system compensates for separation of the physical 

 parts of the body of the organism, maintaining unity for the 

 organism co-ordinate with the physiological specialization 

 and the morphological differentiation. 



Branches, Classes, and Subclasses of Mollusca. — Lankes- 

 ter's division of the Mollusca as a Phylum is first into two 

 branches: 



Branch A, the GlossopJiora, characterized by a "head 

 region more or less prominently developed ; always provided 

 with a peculiar rasping tongue — the odontophore — rising 

 from the floor of the buccal cavity;" and 



Branch B, Lipoccphala, of which the characters are 

 " Mollusca with the head region undeveloped. No cephalic 

 eyes are present; the buccal cavity is devoid of biting, rasp- 

 ing, or prehensile organs. The animal is sessile, or endowed 

 with very feeble locomotive powers." 



All these latter branch characters are practically negative 

 characters: the Glossophora is a group formed of the Mollusca 

 which possess in common a few important characters, and the 

 Lipocephala are those which do not possess those characters. 

 Only one class is recognized in the Lipocephala, i.e., the 

 Lamellibranchia. The Glossophora comprise the three classes : 

 first. Gastropoda, with two subclasses, (i) the Isopleura and 

 (2) the Anisopleura; second, the class Scaphopoda; third, the 



