666 COMPARATIVE ANATOMY 



The absence of bony skeletal structures in the simpler chordates 

 makes it appear unlikely that paleontological evidence can be used in 

 the solution of the problem. Students in the past have possibly depended 

 too much upon comparative anatomy. The study of genetics has not 

 tended to support the orthodox assumption of the transformation into 

 chordates of adult invertebrates of the types now extant. Future stu- 

 dents of phylogenesis will turn to embryological evidence more than has 

 been done in the past. Delsman's suggestion of a common origin of 

 annelids, molluscs, and chordates by alteration in the growth centers of a 

 trochophore-like ancestor sounds plausible. 



For the present, morphologists must resort to the eclectic method 

 used in this text, and select the hypothetical stages in the evolution of the 

 various organ systems in whatever invertebrate types they may be found. 

 However, in view of the evidence of the convergent and independent 

 appearance of similar structures in different phyla, phylogenetic "series" 

 are necessarily speculative and tentative. 



Among the invertebrate groups with traits resembling those of the 

 hypothetical ancestors of vertebrates this text has mentioned Protozoa, 

 Coelenterates, Platyhelminths (including Nemerteans), Nematodes, and 

 Annelids. More closely related to vertebrates than any of these are 

 Hemichordates, Urochordates, and the Cephalochordates, which may 

 therefore be regarded as having some of the ancestral traits. None of 

 these, however, lies in the main line of vertebrate descent, as is indicated 

 by the phylogenetic tree of the animal kingdom shown in Fig. 540. The 

 main line of vertebrate phylogenesis is today better represented in larval 

 stages like the trochophore than in any adult forms. On the whole, 

 the evidence supports the conclusion that the separation of animal phyla 

 occurred farther down in the phylogenetic series than has hitherto been 

 assumed. See Fig. 520. 



