CHAPTER XXI. 

 THE VERTEBRATES. 



We are now in a position to see more clearly the relation that the true verte- 

 brates bear to the ostracoderms. The full recognition of the ostracoderms as 

 the common ancestors of all vertebrates will prove to be a fruitful idea and will 

 go far toward laying at rest the obscession of the last thirty years or more, that the 

 foundations of vertebrate morphology rest on Amphioxus and the elasmobranchs; 

 that in them is the beginning and the end, beyond which lies a fathomless abyss. 



It will be a great step forward if it can be demonstrated beyond a reasonable 

 doubt, as I believe it can be demonstrated, that the heavily armored ostracoderms, 

 with cephalic appendages and a large atrial or peribranchial chamber, form the 

 starting point for all animals entitled to be called vertebrates; not the dermal- 

 denticled shark, nor the naked-skinned cyclostomes, nor the impotent remnants of 

 animals that constitute the acraniates. 



If we attempt precisely to define the natural limits of the ostracoderms 

 and their immediate descendants, we at once meet with great difficulties and 

 after all we must for the present resort to arbitrary definitions. It is doubtful, 

 for example, whether we should, or should not, exclude the ccelolepidae and the 

 anaspidae from the ostracoderms, or whether we should, or should not, include 

 with them the arthrodira, or even the antiarcha. The dipnoi might be excluded 

 from the "fishes" on the ground that they are really primitive amphibians. If 

 so, we should then have remaining, as representative fishes, the elasmobranchii, 

 holocephali, teleostomi and cyclostomes, forms very unlike in structure and origin. 

 However, while recognizing the inadequacy of the current terminology, we shall 

 nevertheless adopt it, as indicated in the tabular scheme of relationships, without 

 attempting to build up a new one that might express more clearly the views herein 

 set forth. 



The ostracoderms may be briefly characterized as follows: They consist 

 of a comparatively small number of metameres, and are provided with a highly 

 developed dermal armor, cephalic locomotor appendages, paired jaws, a large 

 atrial or peribranchial, chamber, and with eyes and olfactory organs located 

 near the middle of the aboral surface of the forehead. The form and general 

 appearance suggest that of a trilobite or merostome, or an amphibian tadpole, 

 rather than that of a true fish. Their negative characters, compared with those 

 of the vertebrates, consist in the presence of a diminutive notochord, without 

 definite constrictions or thickenings of the sheath to form centra, and without 

 recognizable neural or haemal arches, pectoral or pelvic appendages, or teeth. 



It is a remarkable fact that the descendants of the ostracoderms, in their 



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