THE ANCESTRY OF THE VERTEBRATES 



647 



"The evidence of geology points to the same conclusion. Evolution 

 takes place from one dominant stock to another dominant stock — man 

 from mammals, mammals from reptiles, reptiles from amphibia, amphibia 



OBLIQUE MUSCLE 



CEPHALIC STOMACH 



A. 



■ ESOPHAGUS 



PHARYNX 

 PINEAL EVE 



OLD MOUTH 



OLFACTORY NERVE 



OLFACTORY PIT 



COXAL GLAND 



ALIMENTARY CANAL 



B. 



BRAIN VENTRICLE 

 TROCHLEAR NERVE 

 INFUNDIBULUM 

 PINEAL EYE 



SACCUS 

 VASCULOSUS 



OLFACTORY NERVE 



HYPOPHYSIAL TUBE 



FACIAL NERVE 



NEURAL TUBE 



PITUITARY BODY VAGUS NERVE ' NOTOCHORD 



BRAIN VENTRICLE 



NEURAL TUBE 



*-• FACIAL NERVE 



TROCHLEAR NERVE 



PINEAL EYE 

 OLFACTORY NERVE 



NASAL TUBE 



UPPER LIP 

 MOUTH 



PITUITARY BODY VAGUS NERVE NOTOCHORD 



Fig. 526. — Diagram depicting the hypothetical evolution of a vertebrate from an 



arachnid (Eurypterus). A, Eurypterus as seen in median longitudinal section, B, 



Ammocoetes (larval cyclostome), C, Adult Cyclostome (Petromyzon). (Redrawn 



after Gaskell.) 



from fishes. We are therefore forced to conclude that fishes came from 

 pre-Devonian paleostracan arthropods, the ancestors of Crustacea and 

 arachnids. These in turn came from trilobites. The earliest fishes 



