CHAPTER XX 

 THE EVOLUTION OF THE ATYPICAL FISH 



The typical Fish's characters are largely the result of the 

 active locomotive powers possessed by its developing 

 primitive type. But there exists at the present time what 

 may be called the " atypical Fish," and its structural plan 

 would seem to indicate that the primitive type from which 

 it evolved had no motile powers. Examples of atypical 

 Fish are presented by the Mollusca generally, but it is in 

 the class called Cephalopoda that we get the clearest hints 

 as to the way this type of organism evolved. 



Although possessing no vertebral column, and for this 

 reason biologically classified as an Invertebrate, the Cepha- 

 lopod shows no signs of having been derived from primitive 

 Annelid invertebrate stock. In fact, though invertebrate, 

 the Cephalopod probably has a closer relationship to the 

 typical Fish than to the Annelid. Our supposition is that, 

 like the typical Fish, the Cephalopod has been derived 

 from a primitive serially-medusoid form of Continuity, and 

 that the absence of locomotive powers in this ancestral 

 type resulted in the evolution of characters greatly different 

 from those of the typical Fish. 



The serially-medusoid type from which it has been 

 suggested came the typical Fish, moved along actively with 

 its convex end leading, and thus it was that in the evolving 

 Fish the head, mouth, and brain took form in this, so to 

 speak, potential region ; while the potentially concave end 

 developed a tapering form, and the inherited alimentary 

 opening became the anus. All this was due to movement 

 during development, in face of constant water-pressure, 

 and with the convex end leading. 



But if the ancestral serially-medusoid type had not 

 been free-swimming, but had lived attached to the bottom 

 of the sea, can we picture what structural segmental plan 



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