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 cava (all the main blood 

 vessels have contractile walls) ; k, coelom. 



changes are diagrammatically indicated in figure 131. 



The skeleton. In some very primitive allies 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 figure 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 



salamander.) 



FIG. 133. Front end of notocord 



in its relations to the cartilagi- Matprifllc npprlprl T i-Hncr 



nous base of the cranium in the Materials needed . L,\\ ing 



spotted salamander, p, begin- cr>prirnpn<: tn hp pvaminprl fl; tn 



nings of cartilaginous cranium; Specimens tO DC examined as I( 



q . orbit of eye; r. notocord. external features, appearance 



