THE ANCESTRY OF THE VERTEBRATES 657 



case based upon the assumption that vertebrates are descendants of a 

 limulus-like arachnid ancestor. 



Patten's hypothesis is based upon the assumption that ostracoderms 

 are primitive vertebrates. But morphologists are generally of the opinion 

 that ostracoderms are highly specialized descendants of elasmobranchs, 

 more closely related to teleosts than to cyclostomes. 



In order to reconcile his homologies of arthropod and hemichordate 

 embryos, Patten is obliged to ignore the fact that the primary blastopore 

 in hemichordates becomes anus and not mouth. He assumes that the 

 water pore of hemichordates is the primitive mouth or "neurostoma,"" 

 and that the blastopore is a " telopore." Likewise, in spite of the evidence 

 of the origin of the notochord from the primary endoderm of amphioxus, 

 Patten asserts that the notochord is ectodermal in origin, apparently 

 because the lemmatochord of limulus is ectodermal. 



Patten's theory compels him to locate the primitive mouth in the floor 

 of the neural plate, at the place where later the infundibulum develops. 

 That this is a late and secondary development seems not to matter to him. 

 Neither does the fact that embryology presents not the slightest evidence 

 that the foregut was ever open to the exterior at this point. Also, in 

 order to substantiate his hypothesis, Patten is compelled to assume that 

 echinoderms, urochordates, balanoglossus, and cephalodiscus are degen- 

 erate offshoots of a common arthropod-vertebrate stock. The evidence 

 of this degeneration is, to say the least, not convincing. 



Patten's argument for the close blood relationship of arachnids and 

 vertebrates is based largely, if not exclusively, upon details of anatomical 

 structure rather than upon fundamental resemblances. The striking 

 contrast in basic features of the two groups does not accord with expecta- 

 tion, if arachnids and vertebrates are closely related phyla. This diffi- 

 culty has left biologists unimpressed by the mass of evidence of detailed 

 resemblance gathered by Patten. 



Consequently, the evidence of similarity in some, or even many, 

 details of structure fails to convince zoologists of their genetic affinities. 

 Furthermore, the argument that one dominant group of vertebrates has 

 succeeded another dominant vertebrate group does not justify the con- 

 clusion that fishes have come from a dominant group of invertebrates. 

 When two able biologists such as Gaskell and Patten, on the basis of 

 identical material, reach diametrically opposite conclusions, it is not 

 surprising that a skeptical attitude toward phylogenetic speculation 

 characterizes this generation of zoologists. 



The Nemertean Hypothesis 



In 1883, Hubrecht advanced the hypothesis that nemerteans are 

 the long-sought ancestors of vertebrates. Among advantages of this 



