206 



GENERAL BIOLOGY 



rear end of the pronephric duct becomes the ureter; the 

 anterior end, and with it the pronephros, atrophies. These 



Fig. 132. The lancelet (Amphioxus, or Branchiostoma) after Gegenbaur. 

 a, mouth; b, gill chamber; c, sac of enteron corresponding to the liver d 

 stomach; e, intestine' /, anus; g, aortic arch; h, portal vein; i, notocord, 

 with dorsal aorta extended beneath it; ;', post eava (all the main blood 

 vessels have contractile walls) ; k, coelom. 



changes are diagrammatically indicated in figure 131. 



The skeleton. — In some very primitive alHes of the true 

 vertebrates (as for example, the lancelet, fig. 132), the 

 notocord persists through life as the chief supporting struc- 

 ture of the body , but in the salamander as in vertebrates gen- 

 erally, it is early replaced by a cartilaginous skeleton. 

 Its cells become vacuolated, and are finally resorbed 

 and entirely disappear. The beginnings of the develop- 

 ment of the cartilaginous cranium about the front end 



of it are shown for the sal- 

 amander in gure 133. Car- 

 tilage constitutes the whole 

 skeleton of many of the lower 

 fishes, but in the higher verte- 

 brates it is more or less re- 

 placed by bone. 



Study 26. The internal organs 



of an amphibian {frog or 



salamaiider .) 



Fig. 133. Front end of notocord 



'^^^'SiV^.^^.^i'^ Materials needed: Living 



Sngs'of'carTiia"g1nous ?ranS-. spccimcns to be examined as to 

 q. orbit of eye; r, notocord. extcmal fcaturcs, appearance 



