CHAPTER ELEVEN 



The Phylum Chordata 



The most probable relationship of the chordates to the invertebrate 

 phyla has already been outlined in the preceding chapter. But evidence 

 in favor of the echinoderm theory is not conclusive, and almost every 

 invertebrate group has been suggested at one time or another as a possible 

 ancestor of the chordates. Only a few of the more plausible suggestions 

 will be discussed here. 



THEORIES OF CHORDATE ORIGIN 



The Nemertean Theory. Kofoid and Hubrecht suggested that the chor- 

 dates originated from nemerteans because of the arrangement of the 

 nemertean nervous system in eight longitudinal cords. They proposed 

 that the development of the two dorsal cords at the expense of the ventral 

 and lateral cords could explain the origin of the dorsal nerve tube of the 

 vertebrates. While this is plausible, there is no positive evidence in favor 

 of it. As to other systems, the most that can be said is that the nemertean 

 is sufficiently generalized to permit the formation of chordate structures 

 or any others. But there is no evidence that they have specialized in a 

 chordate direction, or that they ever have done so in the past. Much the 

 same can be said for the turbellarian origin of the chordates which was 

 urged by Haeckel. The theory can neither be proved nor disproved, be- 

 cause there is a complete lack of evidence. Similarly, the evidence in favor 

 of the origin of the chordates from the Coelenterata is negative in charac- 

 ter. The coelenterates are so primitive that they could conceivably have 

 given rise to any of the more advanced phyla, including the Chordata. 

 But there is no positive evidence that they did, and this theory has no 

 adherents today. 



The Arachnid Theory. It has also been suggested that an arachnid simi- 

 lar to Limulus might be the ancestor of the chordates. The principal evi- 

 dence for this rather surprising theory is the superficial resemblance 

 between Limulus ( together with its fossil relatives ) and the ostracoderms, 

 the earliest known vertebrate fossils. Yet every detailed study has shown 

 that this resemblance is illusory, and the theory no longer has any ad- 

 vocates. 



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