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a brain in connection with which sense organs were evolved. 

 Probably as a result of this, the anterior end of the body 

 became greatly modified and a well-marked head was formed 

 by the differentiation and partial fusion of some of the 

 anterior segments, including those, possibly reduced in 

 number, in which the gill-slits were present. 



From the Proto-Vertebrata the Cyclostomata were derived. 

 They are probably degenerate forms, and possibly represent 

 what was formerly a large group of primitive Vertebrata. 

 From these the primitive Fishes arose by a series of changes 

 resulting in the forms which Balfour has called the Proto- 

 Gnathostomata. In this group the internal skeleton became 

 considerably increased. Cartilaginous branchial bars were 

 formed in the intervals between the gill-slits. True jaws 

 appeared for the first time in the history of the Vertebrata. 

 The vertebral column and the skull, formed in the meso- 

 derm around the notochord and the dorsal nervous system, 

 became much more completely developed than they were in 

 the Proto-Vertebrata from which the Cyclostomata diverged. 

 Finally, paired fins, or limbs, made their appearance. 

 Probably, as suggested by Balfour and others, these were 

 first formed as continuous lateral fins, one extending along 

 each side of the body from close behind the gill-slits to near 

 the termination of the alimentary canal. Afterwards concen- 

 tration took place at two points on each side, resulting in the 

 formation of the primitive pectoral and pelvic tins, while the 

 part between became suppressed. 



The Proto-Gnathostomata is represented by the existing 

 Elasmobranchii, a group which have probably undergone 

 comparatively little change and are, therefore, of great value 

 as showing many of the typical vertebrate systems in a more 

 or less primitive condition. The Holocephali are the modi- 

 fied descendants of an offshoot from the primitive Elasmo- 

 branchs. 



