276 



EMBRYOLOGY 



In Mitraria we recognized a larva which possesses pro- 

 visional larval appendages in the form of long bristles, 



Fig. 124.— Lateral views of Mitraria larvse (after Mbtschnikoff, from Balfour's 



Comparative Embryology), an, anus ; b and br, the lateral elevations with the pro- ] 



visional setae ; m, mouth; pr. h, preoral ciliated band ; sg, apical plate. ] 



which also occur in other Annelid larva8. Trochophore larvas ' 



are known which exhibit a number of long denticulate j 



bristles on both sides of the bodj, thus, e.g.., in the genera ', 



Sahellaria, Spio, etc. Figs. : 



125 and 126 show larv86 I 



more advanced in develop- ' 



ment with richly developed, '. 



and in part extraordinarily | 



long, provisional setae. Setae 5 



of this kind do not appear I 



in adult recent Chae^topoda, | 



but, on the other hand, are | 



found in fossil forms. It | 

 has been conjectured that 

 they might have been inherited from unsegmented ancestors 

 of the existing Chsetopoda. This suggestion appeared to 

 be supported by the fact that they are mostly found on the 

 anterior unsegmented part of the larva (Alex. Agassiz). 



Fig. 125.— Larva of ^ferine (after Albx. 

 Agassiz, from Balfoue's Comparative 

 Embryology), 



