MORPHOLOGICAL DIFFERENTIATION. 327 



The value of this representation for our purpose is to 

 show the extent of structural elaboration which the evolution 

 of organisms had actually reached at the time when we first 

 meet with a representative of the class Cephalopoda. 



Supposed Characteristics of the Primitive MoUusk. — In this 

 earliest mollusk bilateral symmetry was fully developed. The 

 nervous system was expressed in bilateral pairs of ganglia and 

 nerves. The organs of sense were in pairs: two eyes and two 

 otocysts were present. The body form was normally sym- 

 metrical, its spiral coiling or one-sided development coming 

 as a specialization of growth. The cephalic is sharply distin- 

 guished from the visceral part of the body. The shell is 

 associated with the visceral part, and is not auxiliary to the 

 functions of motion, but is protective in nature. The head — • 

 anterior part — is distinctly connected with motor functions; 

 and organs of the motor and sense functions are separate 

 and widely differentiated. Motion is elaborated into distinct 

 organs for offence and for prehension. 



The alimentary function is dominated by a single central 

 canal, with an anterior mouth, about which are the accessory 

 organs of excretion (nephridia or rudimentary kidneys), and 

 there is a circulatory system, with a heart and a pair of auri- 

 cles and one ventricle. Locomotion is a conspicuous func- 

 tion, and the presence of an enlargement of the mantle as a 

 foot-organ is one of the most characteristic features of the 

 mollusk. 



The differentiation of this foot-organ is also one of the 

 most fundamental of the characters distinguishing the classes 

 of Mollusca, and the adaptation of the part to special modes of 

 locomotion was developed at a very early stage, as indicated 

 by the presence of distinct Gastropoda, Pteropoda, Cephalop- 

 oda, and Lamellibranchiata at as early as Ordovician time. 



Differentiation of the Foot-organ in MoUusks. — This dif- 

 ferentiation is represented in Lankester's diagram of a series 

 of mollusks to show the form of the foot and its regions, and 

 the relation of the visceral hump to the antero-posterior and 

 dorso-ventral axes (Fig. 10 1). In these figures are seen the 

 simple continuous flat foot of the Chiton (i), or isopleural 

 Gastropod, which retains the bilateral structure of the primi- 



