CHAPTER XVIII. 



THE RATE OF MORPHOLOGICAL DIFFERENTIATION IN 

 A GENETIC SERIES; ILLUSTRATED BY A STUDY OF 

 THE CEPHALOPODS. 



The Evidence Furnished by the Cephalopods. — Having used 

 Brachiopods for what they are worth towards illustrating 

 the laws of evolution, another group of organisms may be 

 examined in the same way, to ascertain what they testify 

 regarding the same points of history. 



Cephalopoda present some general peculiarities contrast- 

 ing them with the Brachiopoda. The Cephalopoda are con- 

 structed on a plan which is shared with two or three other 

 large groups of organisms. The class Cephalopoda is, with 

 Gastropoda and Lamellibranchiata, and, according to some 

 authors, Pteropoda, only one of the classes of the branch 

 Mollusca. We are able, therefore, to distinguish its class 

 characters from those of closely allied classes. This we 

 could not do satisfactorily with the Brachiopoda, which stands 

 out sharply distinguished from all other classes of organisms 

 from the earliest geological time. We find the first traces of 

 the Cephalopoda above the first, or Cambrian, period, i.e., 

 we have a well-defined fauna in which no Cephalopoda 

 existed, so far as our records testify. 



Lankester's Schematic Mollusk. — In attempting to introduce 

 a beginner to a knowledge of the Cephalopod mollusk the 

 method of Lankester, so admirably expressed in his article 

 " Mollusca" in the Encyclopaedia Britannica, and afterwards 

 published with others under the title " Zoological Articles, 

 etc," presents some excellent features. 



Professor Lankester constructs a schematic mollusk as 

 represented in Fig. lOO. 



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