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596 



A New Reading for the Annulate Ancestry of the 



Vertebrata 



T 



HE question of the ancestry of the vertebrates being still 

 unanswered, anyone is at liberty to make suggestions. No 

 new facts seem to be forthcoming to enlighten us ; we are driven 

 therefore to find new readings of the old. To those who scorn all 

 theorizing, and are content to wait until the new facts turn up, I 

 would suggest the following questions : Are we sure we have read 

 all that the old facts have to teach us ? Have we arranged them 

 in every possible order, and are we competent to deny that they 

 can yield us any clue to the solution of the problem ? 



I ask these questions somewhat feelingly, because I have 

 recently lighted upon a new way of arranging the old facts, and 

 I propose to offer it to my fellow-zoologists for what it is worth. 

 This much, indeed, I claim for it, viz., that it shows a way of 

 escaping from at least some of the difficulties in the way of the 

 annulate origin of vertebrates. It provides us with another escape 

 from having to assume that the annulate ancestor, with its ventral 

 nerve-cord, turned over on to its back to become the vertebrate 

 with its dorsal nerve-cord ; and it shows how the notochord and 

 neural plate, those most characteristic of all vertebrate structures, 

 might have been unsegmented from the beginning as secondary 

 developments within an originally segmented body. I propose, in 

 short, to show how the assumption of a primitive hirudinean as our 

 ancestral annulate enables us so to re-arrange the old facts as to bridge 

 over the gulf between the invertebrates and vertebrates with start- 

 ling ease. I do not affirm that our nearest invertebrate ancestor 

 was a hirudinean. I only wish to show how it is conceivable that 

 a primitive leech might have developed into a low vertebrate form 

 allied to the cyclostomes. 



My attention was first directed to the birudineans by the fact 

 that the embryonic muscles of cyclostomes and shai'ks are of the 

 same type as are the muscles of the leech. 1 am well aware that 

 histological resemblances are in themselves of no value to morphology. 

 In this case the resemblance served to suggest the hirudineans as 

 the possible annulate ancestors of the cyclostomes. As is 

 well known, they, like the cyclostomes, have no appendages, no 



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