62 



HUMAN BIOLOGY 



fossilized brain casts, were of the primitive vertebrate 

 type; the mouth, gill chamber and digestive tract were 

 beneath the brain and spinal cord and the primary 



Fig. 4. Two of oldest known forerunners of backboned animals. 

 A. Pharyngolepis b. Aceraspis. 



(After Kiaer, from Greorgy, Proc. Amer. Pbilos. Soc.) 



locomotor organs consisted of a closely packed series of 

 zigzag muscle segments on either side of the long axis of the 

 body. Probably also they possessed a notochord or elastic 

 axial rod just below the nerve cord, as do all their less 

 modified descendants. 



But these ostracoderms were, strictly speaking, not yet 

 vertebrates for the reason that they had not yet acquired a 

 jointed bony vertebral column, or backbone. The known 

 ostracoderms, according to the convincingly thorough 

 studies of Stensio, were related to the existing class of 

 cyclostomes, or lampreys and hag-fishes, rather than to the 

 true fishes, within which there is much reason to suppose the 

 line leading to Lind-Iiving vertebrates later arose. 



In the ostracoderms, according to Stensio's evidence, the 

 mouth opening was in series with the openings leading to the 

 gill pouches, as it is in the embryos of all higher vertebrates. 

 Thus we probably should not have tonsils and thyroid and 

 thymus glands any more than we shouki have had a tongue 



