CHAPTER XIX 

 THE EVOLUTION OF THE VERTEBRATE 



The megazooidal precursor of the primitive segmental type 

 through which the highest segmental organisms evolved 

 must not only have exhibited serial megazooidal repetition, 

 but must also have possessed strong locomotive powers. 

 And if we turn to the megazooidal types known to us we 

 find that the only ones distinctly motile are medusoid in 

 plan. It is through these that we can explain the evolu- 

 tion of the present-day typical Fish, and thus picture the 

 evolution of the primitive piscine type from which, without 

 doubt, sprang the higher vertebrates. 



It is clear that an organism like the Fish could not 

 have evolved from a primitive type in which one medusa 

 composed the Individual ; we have to imagine a type in 

 which the Individual would be represented by a series of 

 medusoids ; for a series must be presumed in order to account 

 for the Fish's segmental repetition. Now the only existing 

 medusoid Individuals in whom there is anything approaching 

 a serial medusoid plan are certain members of the Siphono- 

 phora and of the Acraspeda, and it is the latter which throw 

 light on our subject. 



There are two classes of Acraspedote Individual. One 

 is represented by a single large Medusa, and for this reason 

 can' be put on one side ; but the other develops as a great 

 number of medusiform gonophores which are successively 

 set free from the distal end of a long developing series. This 

 class has been already considered under the name of Dis- 

 continuously Megazooidal Individual (page 103), with Aurelia 

 as the example, and the reader is requested to refer to the 

 description already given. 



It is the strobila which interests us, as it represents a 

 long series of medusoids developing in close continuity. 



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